Check system path

[lindonroberts]

In build_interface.import_problem(), check that cache is in the Python path. Easiest way would be to add an ImportError catch to lines 341-342:
return CUTEstProblem(__import__('%s.%s' % (CACHE_SUBFOLDER, problemDir), globals(), locals(), [str(problemDir)]), drop_fixed_variables=drop_fixed_variables)

If this throws an ImportError, just try to import CACHE_SUBFOLDER - an ImportError here indicates the Python path can't see it. This will hopefully avoid issues when using IDEs, etc.

[jfowkes]

This sounds like a sensible way of handling it to me.