Increase sampler test coverage

Currently, due to moving the sampler tests out of the main test script, the coverage reported is pretty low for the samplers. (There's also a less extreme case for the GW plotting which is done nightly). Most of the sampler code is actually tested at every push in a different job.

I'm wondering if there's a way to combine coverage reports from the python-3.7 and python-3.7-samplers jobs.

An alternative would be to write a bunch of unit tests for the samplers.

$ coverage combine may be all we need https://coverage.readthedocs.io/en/coverage-5.2/cmd.html#combining-data-files-coverage-combine