Added tracking for days since the start of the model

The tracking starts from the first day of the weather data, and tracks
if enough days have passed. If enough days have passed, the effect of
tillage will be changed from Tillage to TillageEffectAfter parameter in
the configuration file. TillageEffectDays sets the number of days for
this to happen.

If the effect is not wanted in a model run, it can be disabled by
setting the timeTillageEffects parameter to false. By default, the
effect is in use.