Fix issues due to zero virtual orbitals in a cluster

[basilib]

Resolves a number of issues due to padding and zero virtual orbitals in a cluster.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (b0edcca) to head (26eefad).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
vayesta/ewf/ufragment.py	68.75%	6 Missing and 4 partials ⚠️
vayesta/core/types/wf/fci.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   71.45%   71.45%           
=======================================
  Files         156      156           
  Lines       20876    20891   +15     
  Branches     2951     2954    +3     
=======================================
+ Hits        14916    14927   +11     
- Misses       5093     5096    +3     
- Partials      867      868    +1     

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