Name-based includes

[jtwb]

Issue #211 contains a proposal for name-based includes, where re-usable term lists could be included by name instead of by file path. We decided against making that the standard method for all includes, but right now the Python API still allows file path includes. File path includes don't make any sense for Python API workflows, but name-based includes would fit well.

The proposal here is to remove support for file path includes in the Python API and add support for name-based includes.