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);
+  }
+}