The assumption should really be that the HEAD of the repo is the
current noise reference.  There is therefore no need to store a
special reference hash for the tests.  This simplifies things quite a
bit, in particular by not requiring submitters to update a reference.

This commit drops the reference and modifies the test to just compare
the current code against HEAD by default.  The option to compare
against an arbitrary ref is preserved.

The CI is updated for these simplifications.  The "test" job is now
simple generation of the budgets.  The merge request approval job is
renamed 'review:noise_change_approval`, and it's logic remains mostly
the same: it compares the current budgets against the head of the MR
target branch, and requires the same MR approval to pass.  The overall
MR approval process therefor doesn't change.

This adds a CI special job that runs only on merge requests that update
the test git ref, indicating that a noise has been changed.  The MR will
show present a report of the changed noises, and will block on MR approval.
Once approval has been given, the check_approval job can be re-run, which
should now pass, allow the MR to proceed.

If the noises change but the reference has not been updated, then the CI
pipeline will fail as usual.

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.

to reflect all the recent interface changes

(Gitlab wants it to be called CONTRIBUTING.md)