Add specific feature templates

[lilyminium]

We could add options and templates for:

• subclassing AnalysisBase
• subclassing TopologyBase
• ConverterBase
• and so on

These could be very helpful to developers. There's a bit of discussion already in https://github.com/MDAnalysis/cookiecutter-mda/pull/8

[lilyminium]

As mentioned in #8, more test infrastructure could also be templated in the cookiecutter, including:

• assert_allclose, assert_equal
• pytest.parametrize
• pytest.skipif / pytest.xfail