Add Nuke txReader/txWriter plugin sources and build files

Add source and build code for Nuke plugins to read and write .tx files using OIIO. These were originally written at Luma against a much older version of OIIO, but have been updated to work with the current API.

txReader
-----------
- Reads a single mip level, up-sampling it to the resolution of the 0'th mip level.
- Supports up to 4 channels
- Reads common TIFF and OIIO metadata

**TODO**
- [ ] Support more than 4 channels (need to implement channel name mapping, or just treat all channels after RGBA as single-channel layers)
- [ ] Use OIIO to validate files, rather than TIFF byte testing
- [ ] Implement planar Reader API in Nuke 8?
- [ ] Handle mip orientations properly (if desired)

txWriter
---------
- Writes up to 4 channels
- Provides control over a subset of the `maketx` options
    - Bit depth
    - Filter
    - File "presets" (OIIO, prman)
    - Tile size (unless using preset)
    - Planar configuration (unless using preset)
    - "Fix NaN" mode
    - "Check NaN" option (currently disabled)
    - Verbose/stats output

**TODO**
- [x] Re-enable "checknan" control after testing against OIIO patch #967 
- [ ] Enable support for more than 4 output channels
    - Code for this is already in place, but OIIO does not seem to respect the custom channel names
- [ ] Explore more efficient methods of populating source ImageBuf (e.g. using the Iterator interface)
Thanks for your insights. I'll definitely keep an eye on things here and see if this starts to get used more for 3D rendering in Nuke now that's it significantly outperforms the old version.