Add functionality to run without needing PSRCHIVE

[mlower]

Not everyone has or is able to install the Python wrappers for PSRCHIVE. Better to add functionality so that the RM-fitting function is run on an input numpy array, as opposed to taking a psrchive object as input. Could even save a copy of the polarised spectrum to a textfile?

• Add terminal (argparse) flag so use of psrchive is optional.
• Add check to see if psrchive is installed.
• Add functionality so that use of psrchive objects & functions only run if psrchive is installed.
• Change input of fit_rotation_measure() to take a numpy array as input instead of a psrchive object.
• Save a copy of the polarised spectrum extracted from the psrchive object (optional).