Add ellipticity and Beam Parameter Product calculations

v1.9.0

• add beam_ellipticity()
• add beam_parameter_product()
• rotate x-tick labels when more than 10 ticks
• removed deprecated use of np.matrix()
• M2_report now includes BPP values
• improve API docs
• code linting

v1.8.0

• handle rotated masks properly
• fix readthedoc configuration