Make WebALT grammars modular

In the framework of the WebALT project a gf grammar library was developed for generating simple mathematical drills in a variety of languages. The legal status of this library has recently changed to LGPL, making it suitable to be the starting point for the language services demanded by this work package. To achieve a better degree of interchangeability it is required to organize the existing code into modules, remove redundancies and lay them in a way acceptable for easy lexicon enhancement by way of the grammar developer’s tools of work package 2, WP2.