Allow make_star() to accept a function that generates photospheric spectra

[pscicluna]

If make_star() could feed input parameters to a function, as well as assuming a blackbody or reading from file, this would widen the range of stars that could be treated and be very handy for MCMC/Nested sampling/etc. It would also enable you to pass in a trained emulator from starfish if you wanted. This could be achieved either we adding an arbitrary callable as an optional input to the function, or (when the code is class-y) as part of the init() of the class.

[jontymarshall]

I'll take a look at starfish and see how it works - it'll be trivial to add a third keyword for the 'stype' parameter and have it delve into an emulated model from a function.