add tests which test smaller parts of the code

[jkingslake]

the two tests currently set up test essentially the full code, by loading dat files and processing profiles and loading them into xarrays. More useful tests could just test individual parts of the code, for example, listing all the dat files in a directory and testing that it finds the correct number.

This would be useful for testing because a failed test would be more specific to why something failed (although you do get a traceback).

[jkingslake]

20 added a few more tests. We now have five and some of them do test smaller parts of the code.

Could definitely add more as we go along though.