Minor update - Variogram initial est. improved

This version includes the following minor improvements:

1. Variogram initial bounds for the calibration is set to plausible limits, which should improve the robustness of the calibration. Specifically, the upper bound of:

   • the range is <= half the study area extent (according to the point data).
   • the partial sill is <= 50% of the observed variance/2
   • the nugget is <= 25% of the observed variance/2

2. If fixed head data is input, the data is checked to ensure it contains the columns 'elev' and 'head' (or 'depth').