addition of missing factories
removal of old comments
made the factory class final, removal of old comments
separation in the class hierarchy of the most abstract writers and string writers, that write to files using first a writing to a string
separation in the class hierarchy of the most abstract readers and string readers, that read from files using then a reading from a string
addition of pnml writer + reader for petri nets, by moving the code in dump and load
addition of a new method to create models from files in the factories in preparation of the switch from the load() based method to the method based on the use of readers.
modifications in relation to the new hierarchy of readers / writers (ones operating on strings and others not)
load and dump are not longer implemented since one can use readers/writers for STG models
modification of the Lts package in relation to the new hierarchy of readers/writers, modification of methods to get strings from states/transitions/labels (now require the model in parameter as it may be used, eg for transitions, to compute the state mapping (old states -> new states) for some writers such as aut).