Allow for index arrays in ShermanMorrison

[vhaasteren]

This one solves #319 by allowing EcorrKernelNoise to work with index arrays, rather than slice objects. The function that is changed now is quant2ind, which now by default returns an list of index arrays.

The fastshermanmorrison package with Cython code is also compatible with this change, in case anyone is using that. Just upgrade it with pip

Thanks to Bjorn Larsen for providing a first outline of this modification.

TODO:

• Modify quant2ind to return index arrays (and slices)
• Fix failing tests in test_white_signals
• Change _setup_sparse and _get_ndiag_sparse in EcorrKernelNoise to allow for index arrays
• Update fastshermanmorrison-pulsar for full compliance
• Added unit tests that shuffle TOAs for EcorrKernelNoise

Depends on #352 for codev report

[codecov]

Codecov Report

Merging #356 (b9a3bcc) into dev (2415762) will increase coverage by 0.07%.
The diff coverage is 96.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #356      +/-   ##
==========================================
+ Coverage   88.35%   88.42%   +0.07%     
==========================================
  Files          13       13              
  Lines        3023     3033      +10     
==========================================
+ Hits         2671     2682      +11     
+ Misses        352      351       -1     

Files	Coverage Δ
enterprise/signals/utils.py	86.77% <100.00%> (+0.31%)	⬆️
enterprise/signals/white_signals.py	98.62% <100.00%> (+0.02%)	⬆️
enterprise/signals/signal_base.py	90.14% <94.44%> (+0.03%)	⬆️

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[vhaasteren]

@AaronDJohnson, unit tests for the extra lines are done. As commented on the code, the remaining two lines that aren't covered have never been covered and IMHO shouldn't even exist. I vote to leave those like this