MBCn: `base_kws_vars` copied to avoid global changes

[coxipi]

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• [ ] Tests for the changes have been added (for bug fixes / features)
  • [ ] (If applicable) Documentation has been added / updated (for bug fixes / features)
• [ ] CHANGELOG.rst has been updated (with summary of main changes)
  • [ ] Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Copy base_kws_vars inside the adjust function so that it doesn't change the dict globally.

Does this PR introduce a breaking change?

No

Other information:

[coxipi]

Reassigning a dict in a function works btw:

import copy 
dd = {"a":1, "b":2}
def ff(dd): 
    dd = copy.deepcopy(dd)
    dd["a"] = 2
ff(dd)
dd
>>> {"a":1, "b":2}

[coxipi]

@tlogan2000 I will be merging this once the tests are passed to address the bug you have found, thanks.

[coveralls]

coverage: 89.399% (+0.001%) from 89.398% when pulling d31f51ec73e76b1999a47eeb651dbe29e98721ce on mbcn_params_dict_local into 5c4cf5fa07ca58d21ca6d5824ff899c48db322ac on main.