Fix various issues

[thisac]

• Check types when adding qubits/bits to a circuit and raise a TypeError if an incorrect type is passed to the method.
• Rename label to name for operations.
• Fix inconsistent spacing in primitive object representations. (#17)
• Fix sequential qubit/bit labelling when adding new registers.
• Update operation representations to use repr() for hashables. (#17)
• Adds Circuit.get_bit method (should've been in #23).
• Formats all files using black/isort (mainly affecting operation_generation).
• Ignore operation_generation.py in coverage since it's already indirectly tested in the simulator tests.

Fixes #17

[codecov-commenter]

Codecov Report

Merging #24 (923b1f5) into master (377deed) will increase coverage by 0.25%. The diff coverage is 85.71%.

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@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   90.87%   91.12%   +0.25%     
==========================================
  Files          16       16              
  Lines        1293     1307      +14     
  Branches      207      214       +7     
==========================================
+ Hits         1175     1191      +16     
+ Misses        118      116       -2     

Impacted Files	Coverage Δ
dwave/gate/simulator/operation_generation.py	0.00% <0.00%> (ø)
dwave/gate/circuit.py	100.00% <100.00%> (ø)
dwave/gate/mixedproperty.py	100.00% <100.00%> (+6.45%)	:arrow_up:
dwave/gate/operations/base.py	100.00% <100.00%> (ø)
dwave/gate/primitives.py	100.00% <100.00%> (ø)

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