Amplitude to CI vector conversion

[obackhouse]

Adds routines to convert T amplitudes to C amplitudes and/or CI vectors, and makes some improvements to related parts of the code.

Also adds an exception when trying to convert and REBCC class to a UEBCC class when fourth order amplitudes are present, which needs to consider the separate spin cases even in RHF.

[codecov]

Codecov Report

Attention: Patch coverage is 52.27766% with 220 lines in your changes missing coverage. Please review.

Project coverage is 89.52%. Comparing base (7c01c64) to head (eaf0214).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
ebcc/codegen/GecCC.py	3.05%	127 Missing ⚠️
ebcc/codegen/RecCC.py	51.49%	81 Missing ⚠️
ebcc/ext/fci.py	93.71%	4 Missing and 6 partials ⚠️
ebcc/util/permutations.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   89.61%   89.52%   -0.10%     
==========================================
  Files         101      101              
  Lines      114503   115375     +872     
  Branches      759      781      +22     
==========================================
+ Hits       102611   103285     +674     
- Misses      11658    11848     +190     
- Partials      234      242       +8     

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