WIP: Iris Imager

[bprather]

This is an attempt at writing a ray-traced imaging code directly into KHARMA.

The initial target is "offline" imaging: ray-tracing of existing output files and analytic models e.g. the thin disk. This is more or less working in this PR, although loading could be more straightforward (currently, imaging a dump requires "restarting" from it, which comes with a host of Parthenon idiosyncracies). With some tweaking around parameter parsing, this could be a drop-in replacement for ipole in some contexts.

Eventually, the goal is coincident imaging, integrated into the KHARMA step -- this is both a compelling science case in and of itself (true slow light imaging), and a jumping off point for monte carlo and MOC radiative transfer in KHARMA.