[bugfix] allow access to find_outputs kwarg from make_compound_ray

[brittonsmith]

This fixes the problem associated with Issue #116. make_compound_ray attempts to load the last dataset on the dataset list to do field checking. However, if that dataset isn't present, we get an error. In the instance where not all data is present, the user would want to be able to pass the find_outputs keyword to the LightRay generator to only use outputs that are present. This PR allows one to do that.

I've also bumped the cache versions as a hail mary that it will fix the failing tests.

[brittonsmith]

@chummels, along with PR #49 to yt_astro_analysis, this seems to solve the issues that stemmed from the discussion in Issue #116. You ok with merging this?

[chummels]

Just requesting a minor tweak to the kwarg placement, but other than that, this looks good to me! Thanks for addressing this bug. Once that is changed, feel free to merge.