Added
- Discovery plugin to find PDDL files.
The files are sorted into domain and problem file types based on the contents of the file.
- Tool plugin for the `Validate` application from
[VAL](https://github.com/KCL-Planning/VAL/tree/master/applications#validate).
`Validate` is a utility to check the syntax of PDDL files.
In order to use the tool a domain file is required to have been discovered, and a problem file is optional.