Remove tests as they are not compatible with new API changes to the
Benchcab class. Tests relating to executing PBS commands will be a part
of the PBS wrapper implementation.

The current method of executing subcommands can be improved using a
"parse and dispatch" strategy. Whereby you don't need to run a
conditional to work out which subcommand to use.

This change refactors the Benchcab class so that its "public" methods
can be dispatched easily when inspecting the argparse.Namespace object
returned by the command line parser.

Fixes #196