Minimiser function

[ashleyrback]

Fit class created in #101 should include a method to be passed to any minimiser. This method should be of the form:

def func(self, *args):
    # Match values in args back with the corresponding parameter
    # For each value in args implement the relevant change
    # Combine Spectra to form a single expected spectrum
    # Project onto fit dimensions and flatten
    # Call compute_statistic method of test statistic
    return test_statistic