Parameter conjugate

[Cryoris]

Summary

Adds conjugate to the ParameterExpression. Since they are real, conjugate returns self.
This adds flexibility and convenience, since real numbers in Python (such as float) also support this.

Details and comments

This is analogous to real numbers in Python,

>>> import numpy
>>> numpy.conj(float(1)) 
1.0
>>> numpy.conj(Parameter('x'))  
x  # before: TypeError: loop of ufunc does not support argument 0 of type Parameter which has no callable conjugate method