simplify vertex cover code for unyielded edges. Add support for zephyr or zephyr

[jackraymond]

Two enhancements:
Add support for zephyr lattices on zephyr solvers (Advantage2)
Improve handling of unyielded edges, improving performance for lower yield processors using dwave-ocean-sdk and networkx algorithms.

[randomir]

LGTM, just a few nits below.

[jackraymond]

I think I have implemented all the review suggestions.
Per code suggestion: I bypassed the use of exception catching in the _good_cover routine by calculating the tree width requirement explicitly - there is a bit of untidy (practically irrelevant) redundancy this way but still a big improvement.

[jackraymond]

I neglected to test all of the _good_cover code, this has now been done. Note for next pull request - the tests are currently insufficient and don't execute this function (the full workflow tests work with defect free lattices), need to add new unit tests.
For now I have manually tested all branches.

[randomir]

@jackraymond, some tests are failing because dwave.samplers is not generally available for all versions of python/dimod.

For details see #284, but TL;DR: we need to drop support for dimod<0.12 in order to switch to dwave-samplers/dwave.samplers.

Also, to even get access to Orang / TreeDecompositionSolver (only available as part of dwave-samplers) -- which is a pretty compelling reason.

I'll do that in a separate PR, and then we can rebase this PR.

[jackraymond]

Lazy with my full environment testing!

[randomir]

@jackraymond, with #299 merged, your code should now work after you rebase to master.

[jackraymond]

Done

[randomir]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.87%. Comparing base (9ee89a3) to head (8ae64e1).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
hybrid/decomposers.py	82.85%	6 Missing ⚠️
hybrid/reference/lattice_lnls.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   92.09%   91.87%   -0.23%     
==========================================
  Files          18       18              
  Lines        2252     2264      +12     
==========================================
+ Hits         2074     2080       +6     
- Misses        178      184       +6     

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