WIP: overhaul the gdt expression implementation

Introduce the "inline expression filter" using the
character ":" to mark filters.
WIP: factorization of lexer and parser implementation
WIP: fix problems with gdt.lm implementation

Use variadic functions for stateless expr evaluation.
Do not store levels for numeric variables.
Separate computation of levels and names from model matrix
computations.
Fix problems with plot implementation vs new expression parser
Remove remaining debug print statements in gdt.lm
Implement conditions in gdt plotting functions
Implement string literals in gdt expression parser
Complete implementation of logical operators in gdt expressions
Fix problem with gdt xy plot and enums
More documentation on new gdt functions
Fix implementation of logical operators in gdt expressions