Options can now be selected at
command line without requiring
separate builds of the code

Fixed bug in testbed code

Adjust python api to output slices from
an image so that this can be done from
the command line

Plus additional goodies, such as
- equirectangular camera support
- OpenGL 3.1 support
- more accurate voxel pruning for training
- cleaner GUI

Can also pass datasets, network configs, or snapshots as positional arguments now.

Makes f-theta and latlong modes more meaningful and paves the way for a future Orthographic mode

Courtesy of Eric Haines

- Makes only a small performance difference (~20%) and matches original NeRF behavior. Remove the need for --nerf_compatibility when wanting to draw rough parallels to original NeRF.
- Also adds a GUI slider to adjust cone tracing angle live.

mesh export with python api and run.py support; added PLY export as well as OBJ; added vertex color export (sRGB); fixed exception when loading nerf with camera path json in same folder; added UI when failing to load snapshot or camera path; some UI code consolidation; change render aabb ui to min/max rather than center/rad.

run.py still contained an early, subtly incorrect PSNR computation that has to do with how prior art in NeRF papers (incorrectly) interprets the PNG alpha channel as sRGB-space rather than linear space. This is further complicated by the fact that NeRF's raymarching procedure also operated in sRGB space (rather than linear), which is both non-physical and introduces a subtle discrepancy with traditional PNG background blending. The new run.py code works around all that -- as the code that generated the paper results already did.

The following line of code will now produce a PSNR of ~36.35, matching the paper and reflecting the actual quality of our results.
`./run.py --mode nerf --scene lego/transforms_train.json --test_transforms lego/transforms_test.json --nerf_compatibility`