Add MO-basis RDMs fields

[msricher]

Hi, @PaulWAyers @FarnazH, this is my modification to allow IOData to handle MO-basis RDMs. Thoughts? I will then add a wrapper to PyCI/PySCF and put it in IOData/Scripts, so that we can use IOData with PyCI to run HCI computations.

[codecov[bot]]

Codecov Report

Merging #285 (7defb7e) into master (234ab7d) will not change coverage. The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           master     #285   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files          74       74           
  Lines        8790     8790           
  Branches     1213     1213           
=======================================
  Hits         8351     8351           
  Misses        191      191           
  Partials      248      248           

Impacted Files	Coverage Δ
iodata/iodata.py	100.00% <ø> (ø)
iodata/test/common.py	96.73% <ø> (ø)
iodata/formats/fchk.py	90.51% <75.00%> (ø)
iodata/test/test_fchk.py	100.00% <100.00%> (ø)

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