Python 2 to 3 conversion

[mattpitkin]

This PR coverts the Python files to Python 3. Primarily this has been achieved through using the 2to3 package, although some manual editing has been performed to keep Python 2 compatibility.

I have tested this in a Python 3.8 environment and the following works:

from psrpoppy import populate

pop = populate.generate(1038, 
                        surveyList=['PMSURV'],
                        radialDistType='lfl06',
                        siDistPars=[-1.41, 0.96], # non-standard SI distribution
                        duty_percent=6.,
                        electronModel='lmt85',
                        nostdout=True # switches off output to stdout
                       )

although there may be cases that might still fail.