Switch to including `expnum` in filenames

[wmwv]

Include expnum in stack, claexp, and related filenames. This way one doesn't have to know the expnum to look up an image. The Butler will complete a partial dataId to include the expnum information, which is redundant with field+filter.

1. [x] Switch to repo/test_dr1 instead of tmp/test_dr1 to indicate progress.
2. [x] Remove expnum from scripts calling stack code.
3. [x] Verify processCcd still works
4. [x] Verify coaddition still works
5. [x] Verify subtraction still works.

This is unfortunately going to get mixed in with getting this to work in new w_2018_14, because I need to update to deal with an int size issue that was fixed as of d_2018_04_06.

[wmwv]

expnum is quite working out right for subtractions. The lookup of the partial data Id resolves to the same expnum for all images in the subtraction set. This leads to failed lookups, of course, as the expnum is only correct for one of the image.s

[wmwv]

Leaving subtraction expnum issues to #23.