griddata.m, griddatan.m: Small tweak in input validation. * griddata.m: Only call tolower on METHOD input when it is required. * griddatan.m: Only call tolower on METHOD input when it is required. Add note to documentation that query values outside the convex hull will return NaN.

# The following lines are used to create a minimal version to be used
# as documentation within the GNU Octave GUI widget Qt Assistant.

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HAVE_DOT               = NO
GENERATE_TREEVIEW      = NO
SEARCHENGINE           = NO