Improvements to how NcQcConfig handles dimensions in netcdf file

[jessicaaustin]

When running tests on a netcdf file with NcQcConfig, you must pass in the names of the dimension variables (time, depth, lat, lon, etc) to the constructor.

It would be better to define those dimensions within the config object itself.

And it would be even better if the config could use CF conventions to pick out the dimensions from the file metadata itself.

See #27 (comment) for background and discussion.