Cavity.Calc( gain=True ) - calculate gain required for lasing

[demisjohn]

This would sweep the gain of the Active region, and find where the imaginary part of the guided mode = 0 (or eigenvalue = 1).

May require additional Materials parameter:

myMat = pf.Material( 3.45, active=True )

So Cavity.calc knows which layer's imaginary part to sweep.
It's possible this could also be determined by just looking at the Eigenvalue's real part without sweeping it.

[demisjohn]

Losses are now calc'd in ea21b87.

Instead of sweeping gain, just calc losses, and user can plot the lowest-loss mode?