I saw in the docs that certain types of units (i.e. angles, concentration) need to be modeled as dimensionless. I'm wondering if we could consider changing this? Specifically, I would be looking for:

Angle

• radian, rad
• degree, deg

Concentration

• mass_percent
• ppm
• ppb
• molality (mol / kg)
• molarity (mol / liter)
• mol / m3

If not, no worries. Others:

1. Are there ever times where you will define two categories that have the same units? Example, I wanted to define specific heat capacity which has the same units as specific entropy. If so, could we add that as a category?
2. Can we add emissivity, ε under dimensionless?
3. Can we add a thermal resistance category: kelvin_per_watts, K/W
4. Can we add a thermal resistance per unit area category: square_meter_degree_kelvin_per_watt, m²K/W
5. Can we add a specific volume category: would have all same units as density, just inverse
6. I see mol in the docs, but could we add it to the database file under substance amount category?
7. Can we move watts_per_square_meter_degree_kelvin, W/m²K under a new category: thermal transmittance and also add btu_per_hour_square_foot_degrees_fahrenheit, Btu/(h⋅ft2⋅°F)

I would support adding these units.

Note that for concentration, units of like type should be convertible. I think ppm and ppb would be convertible with each other, but not to (say) mol/kg. So you would need more than one category of units for concentration.