Add theil, mpr and tau to metrics

[LadyChristina]

All Submissions:

• Have you followed the guidelines in our Contributing documentation?
• Have you verified that there aren't any other open Pull Requests for the same update/change?
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Description

Added 3 additional metrics:

• Theil index
• Max power ratio
• Tau-decentralization index

Checklist

New Metric Support Submissions:

• Did you put the metric's script under consensus_decentralization/metrics?
• Did you name the metric's main function of the script compute_{metric name}?
• Did you import the metric's main function to consensus_decentralization/analyze.py?
• Did you add the new metric (and possible parameter values) to config.yaml?
• Did you write unit tests for the new metric?
• Did you document the new metric in the documentation pages?

[dimkarakostas]

Is this sorted? If so you can avoid making a copy and then finding the max each time (which is O(n) ).

[LadyChristina]

No I don't think this dictionary is sorted