afm, uvl and xml transformation/writer testing

[pabpazjim]

Added 48 models to test fm_metamodels transformations, while also including:

• AFM and UVL related fixes due to errors found while testing (both afm and uvl parser libraries must be updated for this version to work).
• Changes in fm_metamodel towards improved comparison between feature models. Two models will be considered equal regardless of the order of its set elements.

[Virako]

There are some errors about afmparser, I unsdertand that we want to use https://github.com/diverso-lab/afm-parser but is not added as dependency.

[Virako]

Fix mypy errors. The problem is that self.model in https://github.com/diverso-lab/fm_metamodel/pull/32/files#diff-4e18c2f8bc0dd6da8ec67de3e120c43d3e4f304e553560a921d3d8e662719bbdR30 can be None, then the line 66 https://github.com/diverso-lab/fm_metamodel/pull/32/files#diff-4e18c2f8bc0dd6da8ec67de3e120c43d3e4f304e553560a921d3d8e662719bbdR66 when we call to self.model.get_feature_by_name could fail. The options are:

• initialize self.model without Optional
• check self.model is not None when try call some FeatureModel method

[Virako]

The tests are great and work!! :+1: