Add unit tests for morphoclass.features.non_graph

[FrancescoCasalegno]

In #46 we removed various submodules of morphoclass.feature_extractors (now called morphoclass.features), i.e. interneurons, pyramidal_cells, feature_extraction.
Therefore, we also removed the corresponding unit tests for these submodules.

However, we don't have any unit tests for the only submodule left, i.e. morphoclass.features.non_graph.

To clarify, it was already a problem before #46, we just noticed the issue there.

Originally posted by @FrancescoCasalegno in #46 (comment)