Add hydrodynamics-related quantities.

[Aehmlo]

This closes #68, #70, #71, and #72.

[iliekturtles]

Thanks for another huge PR! I didn't get to pressure and density yet.

• Mass rate
  • Wrap at 100 characters.
  • Add tests for carat_per_second through ton_per_second. I don't think the pound_per_minute/_hour conversion factors have enough precision. Add another generic parameter to the test function to accept the time unit.
  • Add at least kilogram_per_minute/_hour. Consider a few more like gram_per_minute or _day suffixes for got kilograms and pounds. The full prefix x prefix matrix isn't necessary but I'd like to see a few of the more common combinations.

[iliekturtles]

• Pressure
  • Change centimeter_of_mercury_4 to "@centimeter_of_water_4: ...; "cm H₂O (4 °C)", "centimeter of water (4 °C)", "centimeters of water (4 °C)";
  • Add tests for non-SI units where the force unit exists.
• Density
  • Some of the <sup> tags got messed up with m³.
  • grain_per_gallon conversion factor should be E-2.
  • Add test for non-SI units where the mass and volume units exist.

[Aehmlo]

I've fixed these, and tests for these quantities are now much more robust.

[iliekturtles]

Resolving conflicts in github didn't work as expected so I manually merged the changes into master. Thanks again for all the effort!