ring-CCD and direct ring-CCD

[obackhouse]

Adds the ring-CCD approaches, equivalent to RPA and dRPA. I need to find some test values before I can merge.

• Add tests

[codecov]

Codecov Report

Attention: Patch coverage is 0.16502% with 605 lines in your changes missing coverage. Please review.

Project coverage is 89.12%. Comparing base (45372fc) to head (8613363).

Files with missing lines	Patch %	Lines
ebcc/codegen/UDFwrwCCD.py	0.00%	122 Missing ⚠️
ebcc/codegen/UwrwCCD.py	0.00%	104 Missing ⚠️
ebcc/codegen/UwdwwrwCCD.py	0.00%	85 Missing ⚠️
ebcc/codegen/UDFwdwwrwCCD.py	0.00%	78 Missing ⚠️
ebcc/codegen/RDFwrwCCD.py	0.00%	57 Missing ⚠️
ebcc/codegen/RwrwCCD.py	0.00%	44 Missing ⚠️
ebcc/codegen/RDFwdwwrwCCD.py	0.00%	38 Missing ⚠️
ebcc/codegen/RwdwwrwCCD.py	0.00%	35 Missing ⚠️
ebcc/codegen/GwdwwrwCCD.py	0.00%	21 Missing ⚠️
ebcc/codegen/GwrwCCD.py	0.00%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   89.59%   89.12%   -0.47%     
==========================================
  Files         111      121      +10     
  Lines      116199   116804     +605     
  Branches      781      781              
==========================================
  Hits       104106   104106              
- Misses      11851    12456     +605     
  Partials      242      242              

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