Remove initialization syntax

[lou1306]

Rationale:

• All initialisation constraints can now be described in the assume section
• assume also allows for relational constraints (including with the agents' id)
• The parser for initializers was never that good to begin with

I am also growing weary of undef. It was probably a bad decision to add it in the first place, its semantics are extremely punishing, etc.

My suggestion is:

• Variable declarations (environment, interface, stigmergy tuples) become mere lists of names
• If a variable is unconstrained then its initial value is any integer
• I would just do away with undef

About stigmergy syntax:

• Right now variables within the same tuple are separated by commas, while tuples are separated by semicolons, and they have no "title". E.g., a,b: 0, 0; c; 1
• I suggest to add a variables title and to use parentheses to group tutples, i.e., variables = (a, b), c