Multiple images per run

[bprather]

Image libraries often involve images of the same GRMHD solution or snapshot with many different sets of parameters. ipole should be more efficient about generating such combinations, allowing the user to specify a number of different combinations, and iterating through all of them reasonably efficiently.

In particular, when tracing one geodesic ipole should be able to track emission given a number of different assumptions e.g. M_unit & electron temperature prescription details.

Additionally, when opening a particular dump file, ipole should be able to image it at several inclinations specified by the user, rather than one at a time.

Imaging at multiple frequencies would also be great -- more likely in the latter category of iteration than at once, due to the way ipole traces geodesics.