Molar heat capacity.

[iliekturtles]

Initial work to setup the quantity. Still need to add additional units and tests.

@nakedible Any specific units you're looking for? If you could review I would greatly appreciate it.

[codecov-commenter]

Codecov Report

Merging #263 (eea7bd8) into master (a7a4ccf) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   96.72%   96.76%   +0.03%     
==========================================
  Files          57       58       +1     
  Lines        2691     2723      +32     
==========================================
+ Hits         2603     2635      +32     
  Misses         88       88              

Impacted Files	Coverage Δ
src/si/mod.rs	100.00% <ø> (ø)
src/si/molar_heat_capacity.rs	100.00% <100.00%> (ø)

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[iliekturtles]

Build failures are limited to 1.37 (to be fixed in another PR) and there are no other unit suggestions so I'm going to merge!