yaml parser: investigate use of multiprocessing to parallelise loading YAML

[GraemeWatt]

The Kubernetes pods used in production each have 16 CPUs (16 sockets with 1 core per socket and 1 thread per core). Using the Python multiprocessing package could potentially speed up the parsing of large submissions by parallelising the loop over data tables:

hepdata-converter/hepdata_converter/parsers/yaml_parser.py

Line 83 in 3f0330d

for i in range(0, len(submission_data)):

It looks like an attempt to use multiprocessing.Pool was started in 980dd23 but later removed in 4b1ad68. If successful, the use of multiprocessing could be extended to other parts of the converter code.