QGIS-TIM (transient) display drawdown contours wrt a steady state model

[jasper-hve]

For obtaining drawdown contours I now subtract 2 rasters with head levels and use raster -> extraction -> contour to obtain drawdown contours. Would be nice if this was a standard output of both steady state and transient calculations.

You can use superposition to circumvent this, however the solution is not correct if you want to incorporate the effect of nearby surface water.

[Huite]

TTim by default computes drawdown rather than head levels relative to a datum (NAP).

I consciously chose to superpose the TTim drawdown on the TimML levels to enable transient heads, but it indeed makes drawdown contours more cumbersome to compute.

I think a reasonable option here it to add a checkbox to enable pure drawdown computations instead. I will have to a little bit of research to see whether this introduces inconsistencies, but I don't think it should be too difficult.

[Huite]

While thinking about #145: it might be easiest to add a "drawdown" mode next to a steady-state and transient modes to enable this. That might be the cleanest way of doing this.