Drop power and exponentiation support in PLCgen.

In #847 (closed) 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.