clarify word-based replacement in docstring and style

also fix typo
bugfix: lists have been replaced by positional args
rename: GRSpec.evaluate -> .eval_init and rewrite to use to_python
rewrite get_vars in 3 instead of 14 lines

rm spec.parser.extract_vars, because it isn't used anywhere and duplicates get_vars.
Note that tree.map is interesting, but simple is better than complex.

Also better to place ast functions in spec.ast, instead of spec.parser
simplify issafety using networkx
add subclass LTL_AST of networkx.DiGraph
mv dump_dot, write_pdf, get_vars from functions to methods of LTL_AST
add method: LTL_AST.sub_values

and allow skipping addition of node to graph in Node.__init__
Also, use id(self) at a single place for picking a unique integer for the node.
Store that in self.id for later reference. This allows for setting it to a value different than id(self).

Otherwise each AST node will be tied to a unique node in the underlying networkx.DiGraph, namely its id.
Replacing nodes will require removing that node and reconnecting a newly added node.
This is not necessary and by using self.id it can be avoided.
add method GRSpec.sub_values for syntactic substitution of values of variables
simplify and pythonize spec.form._sub_all
shift helper functions
bugfixes
add methods: LTL_AST.to_gr1c, to_jtlv, to_promela, to_python

that expose the respective methods of the root AST Node.
bugfixes
bugfix: be careful with scope, yacc couldn't see the operator precedence

resulting in 357 shift/reduce errors.
add method: LTL_AST.sub_constants

to substitute values of constants depending on the domain of the variable on the other side of the Binary operator above the Const. Note how substitution happens, which is interesting. From each constant, ascend until finding the first Binary operator, then descend on the other branch until finding the first variable (some more error handling for malformed formulas doesn't hurt to be added in the future). Then use the domain of that variable to decide with what integer to replace the string constant.
rewrite finite_domain2ints to use LTL_AST.sub_constants

merge _sub_vars with finite_domain2ints
unclutter code through refactoring (+ some pythonism and pep8)
comment unused token "PRIME" and unused import

and a shorter line
bugfix: refer correctly to tabmodule file in setup.py