Implement alternative gdt plot functions

New implementation is based on the same expression parser used
for lm linear fit.
Consider enums as optional in plot schema expression
Better factorization of code for expressions eval

Expression evaluation is moved in a separate file and a generic
identifier scope resultion is introduced.
Implement xyplot using table expressions
Fix problem with stat in gdt functions

Keep the complete name of the parameter including the statistical
function itself.
Implement gdt reduce function using gdt expr parser
Switch to new gdt plot implementation based on expressions
Fix error report message in gdt expression parser
Remove CSV testing files