Add system channel for `Radiant Exposure` / `energy per unit area`

[magx2]

Can you please add system channel for Radiant Exposure? The default unit can be

$\frac{J}{\text{m}^2} or \frac{W \cdot s}{\text{m}^2} or \frac{kWh}{\text{m}^2}$

The main difference from intensity ($\frac{W}{\text{m}^2}$) is that radiant exposure contains time in it.

[magx2]