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**
- [x] Support more than 4 channels (need to implement channel name mapping, or just treat all channels after RGBA as single-channel layers)
- [ ] Implement planar Reader API in Nuke 8?
- [x] 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 
- [x] 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)