Changing the values of the Relax_fit_methods C array in the __init__() method was not working, as it
was changing the class methods rather than the instance methods. Instead the tp_getattro property
is being used and the Relax_fit_getattro() function is used for the target function switching. This
allows the target function to be changed via the __getattr__() method. However this method is being
called once for each target function call, hence it is not efficient. This follows from
http://stackoverflow.com/a/27246336/4309197.
