Testing: Clean up creation of objects

[xtrojak]

A lot of testing objects are created in the tests. These are probably often repeated and redundant. This should get cleaned up by for example putting them into a single location and importing them in the tests.

[xtrojak]

This can also include models - these are sometimes as strings, sometimes as files - unify to files.

[xtrojak]

For the objects that are created by parsing we can basically have a separate file, where individual parsers are also available.

model_parser = Parser("model")
complex_parser = Parser("rate_complex")

complex1 = complex_parser.parse("KaiC(S{a}).T{u}::cyt").data.children[0]
...

Just pay attention to namespaces then.

[xtrojak]

This was partially targeted in #85.