Data for tests can be simplified

[tovrstra]

We should first get rid of some big data files before tackling this. See #6

Currently the data used in the tests are spread over three places:

• directories with npz files in gbasis/test/data
• gbasis/test/gobasis_data.py
• gbasis/test/mol_data.py

The latter two can be put in these directories as well. It would also be cleaner and simpler to have just one function to load data, rather than different ones.