User-defined PAD variables in canopy tools

Canopy simulation tools (hemispherical photograph, LAI2000 and transmittance light map) assume the given input voxel file contains a PadBVTotal variable.

PAD variables are now estimated in the R package from attenuation or transmittance variables and have different names. In order to use the canopy simulation tools, user must before-hand rename pad_smthg variables into PadBVTotal.

Update tools so that the PAD variable is selected from a combo-box in the GUI.