add branches in PR template

[Edoardo-Pedicillo]

I would like to introduce this new PR template to force people to specify which branches should be used by the reviewers to test the PRs.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

Merging #445 (2eb7d3b) into main (5cd0b1d) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #445   +/-   ##
=======================================
  Coverage   97.03%   97.03%           
=======================================
  Files          48       48           
  Lines        3107     3107           
=======================================
  Hits         3015     3015           
  Misses         92       92           

Flag	Coverage Δ
unittests	97.03% <ø> (ø)

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