Refactor schema validation for ChemKED files

[bryanwweber]

ChemKED (Chemical Kinetics Experimental Database) files represent experimental data from chemical kinetics experiments in a YAML format. We have defined a schema for these files, but we are having trouble scaling the validation of the files to new formats. We presently use Cerberus (http://docs.python-cerberus.org/en/stable/) to validate the files.

See also: pr-omethe-us/PyKED#103

This project would help the maintanability of the PyKED codebase and make it easier to extend going forward.