Switchable numpy backends

[obackhouse]

Adds a ebcc.backends module which allows the numpy module to be switched. This is specified by the BACKEND variable, reading the EBCC_BACKEND environment variable at import time.

Currently available backends:

• numpy
• tensorflow
• cupy
• jax
• ctf

Unless you're using a numpy backend, not all features are guaranteed to be functional. I've ensured that at the very least CC calculations on simple ansatzes are supported. jax and tensorflow have simple unit tests. ctf may currently only support RCC. I am fine with this as these are experimental features.

Other changes:

• DIIS object is now a custom implementation
• Lots of changes to the way the array API is interacted with to make sure functionality can be delegated to a backend with needing to do hacky patching
• The einsum contraction functions are now handled better and there are some flags to customised them. Could do with some benchmarking.
• Space masking is mostly done by slices now, with warnings for disjoint spaces. This is no loss of functionality since orbital spaces can be reordered to make the spaces contiguous.

[codecov]

Codecov Report

Attention: Patch coverage is 82.13287% with 831 lines in your changes missing coverage. Please review.

Project coverage is 88.61%. Comparing base (2d2a692) to head (6cbcc8c).
Report is 60 commits behind head on master.

Files with missing lines	Patch %	Lines
ebcc/codegen/UCCD.py	72.70%	128 Missing ⚠️
ebcc/codegen/RDFCCD.py	50.27%	92 Missing ⚠️
ebcc/backend/_ctf.py	0.00%	82 Missing ⚠️
ebcc/codegen/RDFCCSD.py	83.60%	82 Missing ⚠️
ebcc/codegen/RCC2.py	82.99%	75 Missing ⚠️
ebcc/codegen/GCC2.py	73.55%	73 Missing ⚠️
ebcc/codegen/RDFCC2.py	83.99%	65 Missing ⚠️
ebcc/backend/__init__.py	22.36%	53 Missing and 6 partials ⚠️
ebcc/codegen/RCCD.py	77.29%	42 Missing ⚠️
ebcc/codegen/GCCD.py	70.92%	41 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   88.79%   88.61%   -0.18%     
==========================================
  Files          88       94       +6     
  Lines      114148   114474     +326     
  Branches      898      954      +56     
==========================================
+ Hits       101353   101439      +86     
- Misses      12603    12832     +229     
- Partials      192      203      +11     

Flag	Coverage Δ
	88.61% <82.13%> (?)

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