Implementing option to limit the calculated trends for trending/detrending.

Implementing option to limit the calculated trends for trending/detrending. If the calculated trend exceeds the limits, the trend model will be recalculated with either the minimum or maximum prescribed value, respectively.

For example, if one wants to limit the temperature gradient between the dry and wet adiabatic lapse rate, one could add:

TA::algorithms      = IDW_LAPSE
TA::idw_lapse::soft = true
TA::idw_lapse::rate = -0.008
TA::idw_lapse::trend_max = -0.006   #Wet adiabatic lapse rate
TA::idw_lapse::trend_min = -0.010   #Dry adiabatic lapse rate

Then, if the calculate lapse rate is smaller than -0.010, or larger than -0.006, it is fixed at -0.01 or -0.006, respectively.

NOTE: Is there a direct way to access the calculate trend from the trend object? Right now, I use the trick to query values with 1 m difference, which provides the: trend trend_model(1.) - trend_model(0.), but if there is a direct way to access the calculated trend, that would be preferred, I think.