[enhancement] Add power law mass ratio options for reweighting

[maxbriel]

Allows the user to request a mass ratio distribution following a (requested) power law slope.
Usage example:

params = {
    'secondary_mass_scheme': 'power_law_mass_ratio',
    'mass_ratio_slope': -1.0,  # alpha in q^alpha
    'q_min': 0.0,
    'q_max': 1.0,
    ...
}

[sgossage]

Should we make n_points an argument vs. hard coded?

[sgossage]

Not strictly necessary for this PR, but it looks like this class has some overlap with FlatMassRatio and we could create a parent class that this class and that class are children of, modifying just the relevant parts.