Add validation of tarball builds

Add validation of tarball builds

Allows validation of GROMACS builds against a file stored in the
release tarball to give users an indication if the source they are using
is actually unmodified from the release version.

This only applies to builds from tarballs, as the git version checking
already does all of this for us.

Hash can be generated by the new cmake target checksum-files, and applies
for files in src/ and python_packaging/, if they are included in
a whitelist defined in the script.

Refs #2128

Change-Id: Ia3bc41c0d0566993166f567409510cf9c9ec2020