New Variable symbol for Symbolic Regression that can handle missing values

[mkommend]

Similar to factor variables a new variable symbol that returns a constant if no value is present or otherwise the variable value times a weight should be implemented.

 if(var.value is NaN) return c;
 else return var.Value * w;

Necessary steps

• Implement symbol and adapt interpreters
• Include and update grammars
• Adapt constants optimization
• Unit test
• New rules for model simplification