tests compare against git hash, eliminate cached .h5

This updates the test infrastructure to store a git hash of the reference
code, instead of generated .h5 files.  The test infrastructure creates a
cache of reference .h5 files from the specified git hash if it doesn't
already exist, and then compares against that cache.

Comparisons against arbitrary git hashes can be specified manually.
The cache curves are generated from a shell script so that it uses the
checkout of the python code at the reference commit.  Multiple caches based
on git references are stored, and old caches are automatically pruned to
keep just the latest five.

This gets rid of the need for git-lfs and the cached .h5 files, requiring
instead the code be checked out from git to run the tests.