Added create_wavelengths and inc_luminosity

[tomasstolker]

• Added the create_wavelengths function to create logarithmically spaced wavelengths at a fixed resolution.

• Added the inc_luminosity parameter in plot_posterior to include the posterior of the luminosity.

• Some changes and improvements to the plotting tools.

[tomasstolker]

• The add_object and get_object do now support the use of duplicate filters (see API documentation). This hasn't yet been propagated into the fit tools.

• I have also replaced colors with plot_kwargs in plot_spectrum to allow more detailed tuning of the photometric fluxes and spectra.