Add support for measurements

[thisac]

Adds support for:

• mid-circuit measurements
• measurement sampling in the simulator and on measurement objects
• expectation value calculations on measurement objects
• conditional operations based on previous measurements

[thisac]

Will add some examples to the examples folder, as well as update the readme so that it takes these changes into account. Also haven't added a reno entry yet, so a few things are left before this is merge-ready.

[codecov-commenter]

Codecov Report

Merging #12 (bf389aa) into master (32285d4) will increase coverage by 0.90%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   89.89%   90.80%   +0.90%     
==========================================
  Files          15       16       +1     
  Lines        1148     1261     +113     
  Branches      179      201      +22     
==========================================
+ Hits         1032     1145     +113     
  Misses        116      116              

Impacted Files	Coverage Δ
dwave/gate/utils.py	100.00% <ø> (ø)
dwave/gate/circuit.py	100.00% <100.00%> (ø)
dwave/gate/operations/base.py	100.00% <100.00%> (ø)
dwave/gate/primitives.py	100.00% <100.00%> (ø)
dwave/gate/tools/samples.py	100.00% <100.00%> (ø)

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