Add dist.sh script to trivialise making a clean tarball

The source distribution for 1.0.13 (see computing/sccb#1113+) is not as pristine as it should be, so to help with future releases I propose here a new dist.sh helper script that can be run from the git repository, but clones a fresh copy of the given git reference to ensure a pristine tarball.

I hope this is welcome, happy to answer any questions. If this is not required/welcome, please disregard.

requested review from @james-clark and @sophie.hourihane

No objection and thanks - this is also needed for conda releases right? I don't think BayesWave actually needs rpms or debs these days (if it ever really did - i'm not sure anyone ever used the OS installs).

approved this merge request

merged

mentioned in commit cf690990