caching

[hayd]

Would like to do a better job of caching all the things, not sure how:

1. dependancies, atm use a DIY csv to see what I've installed (unfortunately don't think I can just use conda here as am pip installing stuff not on conda). Would be nice to do something like pip freeze and then work out what pip freeze would be (if we were to do reinstall with conda + pip)...
2. if dependancies mismatch a little may be worth removing/installing/upgrading those packages rather than wiping entire env...
3. dist1, atm (and I think this is the case with tox too) the build is reinstalled with pip each time and therefore any built files (e.g. cython files) have to be rebuilt even if they are unchanged. This sucks. There should be a better strategy so that the files can be reused.
4. dist2, if running tests with built files they should be run in the dir with the built files (atm I run in the project directory/toxinidir which I think fails - so no pandas/scipy). This is also a bit fishy as if the tests are modified during a ctox run you may run different tests in some envs, unfortunately when pip installing from a zip not all files are copied across (e.g. test dir not in the package even though in zip/MANIFEST).

Note: 3. and 4. should really be solved simultaneously if can think of the right way^TM, manually (not using extract_all as I think this deletes dir first?) unzipping may work?