Enable posterior plots of added cbc parameters

The function construct_cbc_derived_parameters enables us to calculate various quantities for commonly used CBC quantities. This MR allows us to then plot the posteriors in the usual way by adding the new parameters into the list of search_parameter_keys, and also gives them latex labels in parameter_labels. In other words, the following will now create a two-d plot of chi_eff vs chi_p

result.construct_cbc_derived_parameters()
result.plot_corner(parameters=['chi_eff', 'chi_p'])