TilingComposite support for Advantage processor

[jackraymond]

The TilingComposite has been modified to allow tiling of Chimera structured problems over Pegasus as well as Chimera solvers.
This is an important use case for certain users, who wish to compare Chimera-structured (or chimera embeddable) problems between the DW2000Q and Advantage generation processors, at high sample density.
Some minor fixes were also introduced for Chimera case, catching some pathological behaviours.

The DWaveMockSampler has been modified to support Pegasus as well as Chimera structured solver emulation.

These changes allow straightforward generalization for the case of Zephyr topologies.
These changes do not resolve all known outstanding issues with TilingComposite e.g. #295 . This supports tiling of Chimera problems over Advantage (Pegasus-structured) processors, further changes are required to support tiling of Pegasus problems over Advantage (or DW2000Q) processors.

[JoelPasvolsky]

Some comments on the docstrings, I'll look at the code next

(Happy you're doing this; we'll finally be able to dump #294 :-))

[codecov-commenter]

Codecov Report

Merging #431 (d1e6399) into master (10acaa5) will decrease coverage by 3.25%.
The diff coverage is 88.15%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   90.85%   87.60%   -3.26%     
==========================================
  Files          22       22              
  Lines        1455     1492      +37     
==========================================
- Hits         1322     1307      -15     
- Misses        133      185      +52     

Impacted Files	Coverage Δ
dwave/system/composites/tiling.py	91.75% <88.00%> (-4.31%)	⬇️
dwave/system/testing.py	96.36% <88.46%> (-2.58%)	⬇️
dwave/system/samplers/leap_hybrid_sampler.py	61.72% <0.00%> (-13.88%)	⬇️
dwave/system/samplers/clique.py	77.35% <0.00%> (-5.04%)	⬇️
dwave/system/samplers/dwave_sampler.py	84.47% <0.00%> (-3.11%)	⬇️
dwave/system/composites/embedding.py	95.85% <0.00%> (-1.19%)	⬇️

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[arcondello]

Can we add a test for a pegasus processor with missing qubits?

[JoelPasvolsky]

Not due to this PR but I think tiling.py is missing an import warnings statement for

dwave-system/dwave/system/composites/tiling.py

Line 170 in 9d13404

warnings.warn("Incomplete solver topology information."

[jackraymond]

Testing both edge defects and node defects independently for the tiling composite (and other functions) is quite important so I added that functionality to MockDWaveSampler as well.
Other changes on November 8th are superficial - mostly PEP8 stuff, and doesn't impact tiling composite other than an expansion of the tests run.

[arcondello]

@JoelPasvolsky, do you want to finish reviewing?

[JoelPasvolsky]

LGTM, gives good results for all the tests I tried.