adding support for customNonbondedForce and xml script tags

Summary of changes so far:

-added copier function for customNonbonded: currently copies only one global parameter, probably should copy all of them

-added call to addff() in adff_xml() to parse script in xml file: current assumes script is the last element, probably should search over all elements in case there's more than one script or if script is not at the end of the file

-changed writing to newffdata in make(): currently copies text from the temporary .txt file containing the xml script and writes that into the text attribute of the script element in the xml file