This generic hook function is called whenever a grid grob is created
This provides an opportunity for customising the validation of a
new class derived from grob (or gTree).
A grid grob.
The function MUST return the validated grob.
This function is called by
A method should be written for classes derived from grob or gTree
to validate the values of slots specific to the new class.
Note that the standard slots for grobs and gTrees are automatically
gp slots for grobs and,
childrenvp slots for
gTrees) so only slots specific to a new class need to be addressed.