remove stats system

improve operations and make them extendable more easily and let other plugins add operations, like requirements 
move requirements/operations out of ranks class as they will be used in prestiges

src/main/java/sh/okx/rankup/requirements/requirement/GroupRequirement.java

@@ -0,0 +1,34 @@
+package sh.okx.rankup.requirements.requirement;
+
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+import sh.okx.rankup.requirements.Requirement;
+
+public class GroupRequirement extends Requirement {
+  public GroupRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+  }
+
+  protected GroupRequirement(Requirement clone) {
+    super(clone);
+  }
+
+  @Override
+  public boolean check(Player player) {
+    OUTER:
+    for (String requiredGroup : getValueString().split(" ")) {
+      for (String group : plugin.getPermissions().getPlayerGroups(player)) {
+        if (group.equalsIgnoreCase(requiredGroup)) {
+          continue OUTER;
+        }
+      }
+      return false;
+    }
+    return true;
+  }
+
+  @Override
+  public Requirement clone() {
+    return new GroupRequirement(this);
+  }
+}

src/main/java/sh/okx/rankup/requirements/requirement/MoneyRequirement.java

@@ -0,0 +1,40 @@
+package sh.okx.rankup.requirements.requirement;
+
+import net.milkbowl.vault.economy.Economy;
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+import sh.okx.rankup.requirements.DeductibleRequirement;
+import sh.okx.rankup.requirements.Requirement;
+
+public class MoneyRequirement extends DeductibleRequirement {
+  public MoneyRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+  }
+
+  protected MoneyRequirement(Requirement clone) {
+    super(clone);
+  }
+
+  @Override
+  public boolean check(Player player) {
+    Economy economy = plugin.getEconomy();
+    double balance = economy.getBalance(player);
+    return balance >= getValueDouble();
+  }
+
+  @Override
+  public void apply(Player player) {
+    Economy economy = plugin.getEconomy();
+    economy.withdrawPlayer(player, getValueDouble());
+  }
+
+  @Override
+  public double getRemaining(Player player) {
+    return Math.max(0, getValueDouble() - plugin.getEconomy().getBalance(player));
+  }
+
+  @Override
+  public Requirement clone() {
+    return new MoneyRequirement(this);
+  }
+}

src/main/java/sh/okx/rankup/requirements/requirement/PlaytimeMinutesRequirement.java

@@ -0,0 +1,41 @@
+package sh.okx.rankup.requirements.requirement;
+
+import org.bukkit.Statistic;
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+import sh.okx.rankup.requirements.Requirement;
+
+public class PlaytimeMinutesRequirement extends Requirement {
+  private static final int TICKS_PER_MINUTE = 20 * 60;
+  private Statistic playOneTick;
+
+  public PlaytimeMinutesRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+    try {
+      playOneTick = Statistic.valueOf("PLAY_ONE_MINUTE");
+    } catch (IllegalArgumentException e) {
+      // statistic was changed in 1.13.
+      playOneTick = Statistic.valueOf("PLAY_ONE_TICK");
+    }
+  }
+
+  protected PlaytimeMinutesRequirement(PlaytimeMinutesRequirement clone) {
+    super(clone);
+    this.playOneTick = clone.playOneTick;
+  }
+
+  @Override
+  public boolean check(Player player) {
+    return player.getStatistic(playOneTick) * TICKS_PER_MINUTE >= getValueDouble();
+  }
+
+  @Override
+  public double getRemaining(Player player) {
+    return Math.max(0, getValueDouble() - (player.getStatistic(playOneTick) * TICKS_PER_MINUTE));
+  }
+
+  @Override
+  public Requirement clone() {
+    return new PlaytimeMinutesRequirement(this);
+  }
+}

src/main/java/sh/okx/rankup/requirements/requirement/XpLevelRequirement.java

@@ -0,0 +1,36 @@
+package sh.okx.rankup.requirements.requirement;
+
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+import sh.okx.rankup.requirements.DeductibleRequirement;
+import sh.okx.rankup.requirements.Requirement;
+
+public class XpLevelRequirement extends DeductibleRequirement {
+  public XpLevelRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+  }
+
+  protected XpLevelRequirement(Requirement clone) {
+    super(clone);
+  }
+
+  @Override
+  public boolean check(Player player) {
+    return player.getLevel() >= getValueInt();
+  }
+
+  @Override
+  public void apply(Player player) {
+    player.setLevel(player.getLevel() - getValueInt());
+  }
+
+  @Override
+  public double getRemaining(Player player) {
+    return Math.max(0, getValueInt() - player.getLevel());
+  }
+
+  @Override
+  public Requirement clone() {
+    return new XpLevelRequirement(this);
+  }
+}