DiffusionCoefficient quantity with basic units "square meter per second" [m2 / s] will be useful for describing

1. diffusion processes, and also
2. kinematic viscosity, that has similar physical meaning.

Proposed units of measurement are:

1. square_meter_per_second and square_[centi | milli | micro| nano]meter_per_second
2. stokes (cm2/s) and centistokes (mm2/s), used for kinematic viscosity

The proposed draft is in PR #322