Enhance support for Zephyr topology

[seatim]

Overview of changes:

• Add support for Zephyr topology and new solver property per_group_coupling_range to DWaveCliqueSampler
• Introduce dwave.embedding.zephyr module
• Minor unit test improvements

Note: not included in this PR are:

• support for Zephyr topology in TilingComposite

[codecov-commenter]

Codecov Report

Merging #448 (22415fe) into master (791f3b2) will decrease coverage by 3.96%. The diff coverage is 17.24%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
- Coverage   90.52%   86.56%   -3.97%     
==========================================
  Files          22       23       +1     
  Lines        1520     1533      +13     
==========================================
- Hits         1376     1327      -49     
- Misses        144      206      +62     

Impacted Files	Coverage Δ
dwave/system/coupling_groups.py	12.50% <12.50%> (ø)
dwave/system/samplers/clique.py	74.35% <23.07%> (-8.04%)	:arrow_down:
dwave/system/samplers/leap_hybrid_sampler.py	61.42% <0.00%> (-14.29%)	:arrow_down:
dwave/system/samplers/dwave_sampler.py	84.75% <0.00%> (-2.44%)	:arrow_down:
dwave/system/composites/embedding.py	96.00% <0.00%> (-1.15%)	:arrow_down:

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[boothby]

this should probably be a PR to minorminer, as the chimera.py and pegasus.py have been reduced to stubs importing from there.

edit: that was hasty; this comment only applies to the zephyr.py file.