[ENH] Add Ensemble implant

[tallyhawley]

Description

Adds the EnsembleImplant which extends ProsthesisSystem. This class is meant for simulating multiple implants meant to be used in tandem, such as CORTIVIS, but can be used with any implant type. It takes a list or dict of ProsthesisSystem objects and combines their electrode arrays.

If the input is a list, the new electrodes are named in the form idx-electrode_name where idx is the index of the implant in the list.
If the input is a dict, the new electrodes are named in the form key-electrode_name where key is the key of the implant in the dict.

Closes #527

Type of Change

• New feature (non-breaking change which adds functionality)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov-commenter]

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