Bug fix in plotMeanField

Object 'mat' defines the plotting window layout, but I accidentally deleted the line before latest commit
Documentation update
NAMESPACE update
Minor improvements in loadMultiField

Also r-oxygen header completed
Horizontal legend in plotMeanField

Allows better visualization and does not cut the numbers
Improvement in on-screen error message

Indicates explicitly the name of the variable not found, and provides instructions on how to check correct naming
Bug fix in interpolation method choices

Other minor improvements
Minor changes