Fix hash function for 64 bit units base

[SimonHeybrock]

With -DUNITS_BASE_TYPE=int64_t the hash function only hashed the first 32 bit of unit_data. This led to identical hashes for different units.

See added unit test, which should fail before the fix put always passed for 32 bit unit_data width.

Fixes scipp/scipp#1829.

[codecov-commenter]

Codecov Report

Merging #150 (bc5a381) into master (04f4006) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files           6        6           
  Lines        4808     4808           
=======================================
  Hits         4772     4772           
  Misses         36       36           

[phlptp]

Thanks,
Seeing this makes me realize I need to add a static assert that the UNITS_BASE_TYPE is either 4 or 8 bytes, otherwise this hash has the potential to seg fault.