In #847 it was decided that `pow(a, b)` and `cbrt(a)` support should be dropped for PLCgen, since: - PLCs don't support `cbrt(a)` but `cbrt(a)` is equivalent to `pow(a, 1/3)`, - PLCs don't support `pow(a, b)` with `a < 0`, even with `b` an non-negative integer value, - Use of `pow` with a non-integer value `b` is rare in control applications, and - Use of `pow` with an integer value `b` can be rewritten to a number of multiplications. Addreses #847