Create a gen_pnet_state module

[joergen7]

Currently, the gen_pnet module exports a number of functions for constructing and accessing the state record of a Petri net instance. These functions are new/2, get_ls/2, get_usr_info/1, and get_stats/1.

These functions should be moved to a new module with the name gen_pnet_state. Also, the name of the state record should match the name of this module (currently it's called net_state).

When this is done, a state instance can be accessed in an OO-style. E.g., the user info field could be extracted by writing NetState:get_user_info().