Right units for trapping equations

[RemDelaporteMathurin]

This PR introduces units (@shimwell), or at least requires users to use units in when dealing with traps.

Because the trapping and detrapping rates expect concentrations (in at/m3) and boxes work with inventories, we had to do some conversion.

When using traps:

• inventories are in kg
• trap densities are in #/m3

Always:

• time is in s