Quantity as an actual class, rather than factory

[khaeru]

Closes #27.

[codecov-io]

Codecov Report

Merging #37 (0a8d284) into master (c79a727) will decrease coverage by 0.13%.
The diff coverage is 98.37%.

@@             Coverage Diff             @@
##            master      #37      +/-   ##
===========================================
- Coverage   100.00%   99.86%   -0.14%     
===========================================
  Files           36       36              
  Lines         2111     2217     +106     
===========================================
+ Hits          2111     2214     +103     
- Misses           0        3       +3     

Impacted Files	Coverage Δ
genno/tests/core/test_sparsedataarray.py	100.00% <ø> (ø)
genno/tests/test_computations.py	100.00% <ø> (ø)
genno/computations.py	98.87% <92.00%> (-1.13%)	⬇️
genno/core/attrseries.py	99.26% <96.96%> (-0.74%)	⬇️
genno/core/quantity.py	100.00% <100.00%> (ø)
genno/core/sparsedataarray.py	100.00% <100.00%> (ø)
genno/testing.py	100.00% <100.00%> (ø)
genno/tests/core/test_computer.py	100.00% <100.00%> (ø)
genno/tests/core/test_describe.py	100.00% <100.00%> (ø)
genno/tests/core/test_quantity.py	100.00% <100.00%> (ø)

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