Fix warning that claims mismatch between DR15 and MPL-7

[albireox]

Describe the bug
This warning shows up when trying to open a DR15 file

In [8]: cube = Cube('8485-1901')
fullpath /Users/Brian/Work/sdss/sas/dr15/manga/spectro/redux/v2_4_3/8485/stack/manga-8485-1901-LOGCUBE.fits.gz
/Users/Brian/Work/github_projects/Marvin/python/marvin/tools/cube.py:186: MarvinUserWarning: mismatch between file release=MPL-7 and object release=DR15. Setting object release to MPL-7
  MarvinUserWarning)

To Reproduce
Steps to reproduce the behavior:

1. Set release to DR15
2. Switch off any DB
3. Open a Cube that exists in your local SAS using its plateifu.
4. Enjoy a warning

Expected behavior
No warnings

Additional context
We can use the aliases in the datamodel to identify releases that are the same.

[havok2063]

These will be explicitly different files but the same entry in the database. They both share v2_4_3 as the version.

[albireox]

Yes, I fixed this by comparing the version string instead of the release. This only matter when opening from file because then you can open a file from a release when setting the release to a different one.