Update name of cubes from gradient module

[mspelman07]

EPP ticket: https://metoffice.atlassian.net/browse/EPPT-1826
Acceptance_test_data: metoppv/improver_test_data#60

Updates the name of output cubes from the gradient-between-adjacent-modules so that the two output cubes can be distinguished.

Testing:

• Ran tests and they passed OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.38%. Comparing base (84a8944) to head (a8b406c).
Report is 33 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
- Coverage   98.39%   98.38%   -0.01%     
==========================================
  Files         124      133       +9     
  Lines       12212    13017     +805     
==========================================
+ Hits        12016    12807     +791     
- Misses        196      210      +14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[seshukolusu16]

I am happy with changes.

[Katie-Howard]

I've looked through this and it all looks fine to me :) I've run the unit and acceptance tests using the new acceptance test data and they all pass too. So I'm happy with this 👍

The only problem is that apparently not all the checks run apparently?