GCalc: Add support to parse unknown variables using $n notation
Currently GCalc can parse named variables and resolve them using the EquationManager, when they were defined as a value or another math expression, use other variables is possible.
Unknown variables, can used:
- When the user wants to provide the value before the expression is evaluated
- When the variable can be set for another process or source, on the fly before the evaluation runs, like when a set of values wants to be pushed to get a set of results for each provided value.
This feature can be used by esodan/libvda#7 (closed) to provide way to substitute values at evaluation of an SQL expression (GCalc provides Math expression parsing).