Add unit test for ensembl.io.genomio.genome_metadata.dump module

[JAlvarezJarreta]

Updated all methods (review docstrings, improve readability and reduce code complexity) and added unit tests for all of them.

In the case of get_genome_metadata() the rework whilst may not be as efficient as the previous code, I believe it is more readable and allows for further meta table checks in case the data found there does not match the expected format, e.g. is species meta key always has a subkey but there is an entry without, a ValueError is raised.

Added to __all__ one of the functions that was missing.

[MatBarba]

Looks good, some questions about the code

[JAlvarezJarreta]

@MatBarba asked for a re-review due to the addition of updated and unit test for filter_genome_meta() (messed up the branching/commits and was unable to push review updates in this PR without pushing this as well). Sorry!

[MatBarba]

Looks good now. At some point we might want to test with test databases, but that part is pretty "simple" so the testing is clear.