Clean up ham module

[obackhouse]

Wasn't happy with the support for external arrays in ham classes. This should be done by faking the object on the calling side as it has no direct use inside ebcc.

Also makes AO basis slicing possible (not yet optimised).

[codecov]

Codecov Report

Attention: Patch coverage is 83.20611% with 22 lines in your changes missing coverage. Please review.

Project coverage is 89.68%. Comparing base (c7dfb7e) to head (8895941).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
ebcc/ham/base.py	74.00%	12 Missing and 1 partial ⚠️
ebcc/cc/base.py	64.28%	3 Missing and 2 partials ⚠️
ebcc/ham/eris.py	91.17%	2 Missing and 1 partial ⚠️
ebcc/ham/fock.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   89.66%   89.68%   +0.01%     
==========================================
  Files          94       94              
  Lines      112772   112720      -52     
  Branches      665      658       -7     
==========================================
- Hits       101115   101088      -27     
+ Misses      11450    11428      -22     
+ Partials      207      204       -3     

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