Symbol refactor fix
ModelHelper.toMathObject(IExpr) -> ModelHelper.toExpression(IExpr)
#72 fix for number * symbol and symbol ^ -number
Better performance
#79 fix and #72 partially fix
No such unary
#72 example 4 fix
#72 example 4 fix
The 'n' variable is now correctly recognized, fixes #73.
Merging Folkert's code fixes #79.

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>