Add IModelRunner interface to TOSA Reference Model

* Added IModelRunner interface using pimpl idiom, which allows a user to
  initialize, configure and run the model.
* Added unit tests for IModelRunner.
* Added doctest as third-party submodule.
* Added user options to specify paths for dependencies.
* Moved general func_config functions to separate utility, which removes
  cxxopts dependency.

Signed-off-by: Matthew Sloyan <matthew.sloyan@arm.com>
Change-Id: If42f1f82cd6dadf18911a48dcd5fa579b719aff2