Model builder fix

[Victorious3]

Fixes #70

• Multiple base classes aren't carried over to the generated python code, instead @tatsumasu only gets the first one (without any ::) This is intentional
• synthesize was receiving string parameters as base classes instead of type objects
• It's unclear what is going to happen when multiple base classes are specified

It might be a good idea to synthesize classes with a directive, i.e something like

@@model :: B(C): a, b   # B is subclass of C and has fields a and b
@@model :: A(B): c      # A is subclass of B and has field c (inherits a, b)
@@model :: D(E, F): a   # D is subclass of E and F (multiple inheritance)

Then only one argument to the rules would be accepted

rule::D = ();  # D does already inherit E and F, no need to specify it here

With these changes the model could be part of the generated parser as well.

I also found that the generated model (python source code) isn't of much use if you rely on the base classes being defined elsewhere. It would need a parameter to add an import statement or some other way to inject them (importlib). - This is now part of the PR

Changes

• There is now an extra parameter base_type for to_python_model which corresponds to the one you can pass to ModelBuilderSemantics, this feature was previously unavailable for generated models
• Tatsu's CLI got an extra parameter --base-type that exposes the above
• The generated model builder gets two optional parameters, types (takes precedence over generated types if specified) and context that are passed on
• You can now safely specify a base class inside your grammar using rule::Class::BaseClass

[Victorious3]

The Model Renderer only accepts one base class and ignores the others, where the one that synthesizes classes on the fly would (if it didn't error out) deal with multiple base classes. This is inconsistent. @apalala What was the intention here?

[Victorious3]

I've decided to go for single inheritance now, i.e
A::B::C means "A is a subclass of B and B is a subclass of C"

[Victorious3]

I'm going to let it generate the base classes as well, then this should be ready for merge.

[apalala]

@Victorious3 This is a complex set of changes, so I won't review them in detail. I'll test for regressions, though:

1. make test
2. run tests for complex Grako projects ported to TatSu, like COBOL.

[apalala]

LGTM!