improved test suite requires new features

[shimwell]

Currently the test suite checks some of the classes and their functionality.
I would be nice to include some additional checks on the material cards to ensure they are created correctly

@RossWorrall has a nice suite of materials which we can incorporate into the test suite.

Unfortunately these materials are only available in mass fraction and MCNP format
and isotopes inside the material cards have been combined/squashed together where possible.

Therefore the material_maker needs to be able to write material cards in MCNP format, in mass fraction and squash identical isotopes. Currently Serpent and atom fraction are supported.

I have started adding the additional functionality to the feature_squash_isotopes_in_material_card branch

New features require isotopes_mass_fraction properties for each class and the changing the serpent_material_card function so that is is more general.