Build conda channel for hepdata_lib

[clelange]

Several people run into issues installing hepdata_lib even though several different methods are provided. There are often problems with inconsistent setups of ROOT and python, but also PyYAML, so adding a conda channel that makes use of the already existing ROOT one might and fixing the PyYAML version to something we know to work could make things bulletproof.
Docs at: https://conda.io/projects/conda-build/en/latest/index.html

[matthewfeickert]

@clelange @GraemeWatt I'll add a PR to https://github.com/conda-forge/staged-recipes to create a feedstock once conda-forge/staged-recipes#26279 is merged (creating https://github.com/conda-forge/hepdata-validator-feedstock). Conda packages don't have the concept of extras, but PR #259 means that a hepdata_lib recipe won't require people to have ROOT (but they can still get it from conda-forge if they want) which is useful for making packaging easier as you don't have to worry about lower bounds on ROOT versions anymore.

[clelange]

Thanks, @matthewfeickert !