Rename ExternalFunction --> Plugin, and implement plugin loading.

src/org/nwapw/abacus/number/externalFunction.java

@@ -1,28 +0,0 @@
-package org.nwapw.abacus.number;
-
-import java.util.HashMap;
-
-public abstract class externalFunction {
-
-    private HashMap<String, Function> functions;
-
-    public externalFunction(){
-        functions=new HashMap<>();
-    }
-
-    public boolean hasFunction(Function x){
-        return functions.containsKey(x);
-    }
-    public Function getFunction(String x){
-        return functions.get(x);
-    }
-    public boolean registerFunction(String x, Function y){
-        if(!functions.containsKey(x))
-            return functions.put(x,y)==null;
-        return false;
-    }
-    public Function functionFor(String x){
-        return null;
-    }
-    public abstract void load();
-}