added the necessary updates for including a central_bh in CMC

[carlrodriguez]

work from Tomás and me on including central massive black holes (MBHs) in CMC.

This places an MBH at the 0 index of the Singles array. When using the regular CMC sampler the mass is specified in solar masses; when using the cmc-point-mass sampler the mass is specified in number of stars (e.g. for a 10,000 point-mass cluster, central_bh=1000 yields an MBH with 10% of the cluster mass).

There's an additional parameter, scale_with_central_bh, which can be passed to the sampler; if True, this adds the MBH potential energy into the calculation when rescaling the stellar positions (default is False).

finally, added a ``seed'' parameter to cmc_point_mass to create different point-mass realizations of clusters.

[codecov[bot]]

Codecov Report

Attention: 357 lines in your changes are missing coverage. Please review.

Comparison is base (8772c07) 86.91% compared to head (337b954) 87.62%. Report is 25 commits behind head on develop.

Files	Patch %	Lines
cosmic/src/assign_remnant.f	9.39%	193 Missing :warning:
cosmic/src/zfuncs.f	6.67%	84 Missing :warning:
cosmic/src/mlwind.f	4.76%	20 Missing :warning:
cosmic/src/hrdiag_remnant.f	64.00%	18 Missing :warning:
cosmic/sample/sampler/cmc.py	22.22%	14 Missing :warning:
cosmic/plotting.py	0.00%	13 Missing :warning:
cosmic/src/evolv2.f	37.50%	5 Missing :warning:
cosmic/checkstate.py	25.00%	3 Missing :warning:
cosmic/src/checkstate.f	0.00%	3 Missing :warning:
cosmic/sample/initialcmctable.py	84.62%	2 Missing :warning:
... and 2 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #613 +/- ## =========================================== + Coverage 86.91% 87.62% +0.71% =========================================== Files 40 42 +2 Lines 25542 25744 +202 =========================================== + Hits 22198 22557 +359 + Misses 3344 3187 -157 ```

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[katiebreivik]

Thanks for adding in the changelog @carlrodriguez !