Updates to documentation, CI, build instructions
- update libtorch requirement to 1.5 or above (works with LCG_99; this is not built by default nor actively used, as far as I know, so should be safe; the only incompatible difference is a class becoming a typedef, but that changes the forward declaration and breaks the build)
- make LCG_98python3 the default in CI (used for documentation build); LCG_97python3 is still there
- add LCG_99 to CI (we could even make this the recommended one, ROOT 6.22/06 instead of 6.22/00 and python3.8)
I will leave this open to pick up more documentation and/or build changes for conda #65 (Tensorflow support there would be nice; lwtnn and ONNX-Runtime are not packaged, so those will be for later) and/or the 1.0 release #66 (closed) .