Raise ValueError when setting chem potentials with mixed type but species duplicates

[lbluque]

Summary

In #113 @juliayang pointed out that setting the chemical potentials of the vacancies with a string as a key was not changing the values being sampled in the ensemble.
This was indeed a bug that happens when the chemical potential dictionary being used has a species duplicate from using mixed types as keys. In the example notebook the duplicate was having a Vacancy object and a "Vacancy" string as different keys in the dictionary, thus when updating the value for "Vacancy" it was being ignored since only the value for Vacancy was being used (or viceversa, whichever comes up last in the dict.items())

This PR simply raises an error when it finds duplicate species in the dictionary being used to set the values of chemical potentials or fugacity fractions.

Additional dependencies introduced (if any)

TODO (if any)

Checklist

• Code is in the standard Python style.
  Run pycodestyle and flake8
  on your local machine.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.

[lbluque]

@juliayang can you have a quick look at this. The only pertinent file should be smol/moca/ensemble/semigrand.py in the chemical_potential.setter

[juliayang]

Good addition @lbluque! It's working as expected.