Rename classes below OperatorVariable to -Parameter

Renames:
  OperatorVariable to   OperatorParameter
     NamedVariable to      NamedParameter
RestrictedVariable to RestrictedParameter
   NumericVariable to    NumericParameter
      PairVariable to       PairParameter

The name 'Parameter' describes better what these classes do.

Affected files:
ast.cpp, ast.h, operator.h, parser.cpp, tree_build.cpp.