TOSA reference model v0.50.0 release tag

- Note that in this release, the specification has added the concept of
levels, but the reference model and conformance tests do not support
testing of levels
- Added FP16 and BF16 data type support, renamed FLOAT to FP32
- Started support for TOSA Main Inference conformance. Still a work in
progress
- Added the IModelRunner interface to allow running the reference model
via an API
- Added simple testing against Numpy
- Tests created/fixed: prelu, CAST, RESCALE, TILE, SIGMOID
- Upgraded Eigen submodule to version 3.4.0
- Misc. bugfixes