Making it possible to compile without Embree enabled

[Waqar-ukaea]

Not really a required PR but I feel like it makes sense from a place of wanting to keep things organised. There are a number of files which are embree specific which have currently been living in include/xdg or src when they could be moved to their respective embree/ directory.

The only changes here are moving files to embree/ and changing headers/Cmake configuration to respect these changes. So from git's perspective lots of "renamed" files since their paths have changed.

[Waqar-ukaea]

I started this PR just trying to re-organise the structure of the Embree specific headers and source files but I then started taking it further by making the changes necessary to see if I could get XDG to compile without Embree enabled. This is proving to be a little more difficult than I had hoped as the GPRTRayTracer class is still missing a lot of the features that the EmbreeRayTracer class has.

I think it would be worth spending some more time getting the GPRT implementation up to speed before attempting to try and make it possible to compile XDG without Embree. Either way, I think there are a few nice QOL changes I've made in this PR which are separate enough to be split out into smaller PRs.

[gonuke]

Although it seems like this is still on hold (maybe?), everything that is here seems pretty logical and valuable. I made a single comment about indentation (for readability) in the CMakeLists file.

[pshriwise]

Made a few more updates here so that tests which can only be run with the Embree backend are skipped when built with GPRT. This won't be evidenced by the current CI setup, but after #183 goes in I'll extend the CI matrix further to configure with Embree only , GPRT only, and Embree + GPRT

[Waqar-ukaea]

Couple suggests/minor points but i'm happy with the additional changes made here.