unittests that need creating

[simonwmatthews]

Most of the module requires unittests still. At the moment I am using this to list features which need tests which I might forget about:

1. The numerical differentiation routines in the default lithology class. These have never been tested, though almost identical code works in the hydrous_lithology class

Prioritise other functions, e.g. behaviour of Mantle class. List work in progress here:

1. Testing individual lithologies - Done (though not quite 100% completion- maybe redundant code)
2. Mantle class - Done
3. GeoSettings - Done
4. Chemistry (including the chemistry method in MeltingColumn) - Done
5. Hydrous lithologies - Done

Completion is not 100%, but all commonly used functions have tests. There may be redundant code.

[kev-wong-geo]

Unit tests for chemistry have been completed. Unit tests for lithologies have been corrected such that McKenzie now no longer causes an error.