Some more Assoc functions
Showing exponential terms as e^(...) instead of exp(...)
UPN parser added.

Example:

    "a 3 * 4 + arsinh eu x ^ +"

is parsed as

    arsinh((a * 3.0 + 4.0)) + e^(x)

or, in native syntax:

    (Sum (Trigo Arsinh (Sum (Product (Variable 'a') (Number 3.0)) (Number
    4.0))) (Exp (Variable 'x')))