Allow return of full FCI vector

[obackhouse]

The FCI vector returned in the conversion routines is truncated to the particular maximum order. This is good for scaling but bad for interoperability with PySCF. This PR adds a keyword argument that can return the full vector, where the high order excitations will be zero (false by default).

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.59%. Comparing base (45372fc) to head (c4873b9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ebcc/ext/fci.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   89.59%   89.59%   -0.01%     
==========================================
  Files         111      111              
  Lines      116199   116202       +3     
  Branches      781      782       +1     
==========================================
+ Hits       104106   104107       +1     
- Misses      11851    11852       +1     
- Partials      242      243       +1     

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