Add test framework

[reubenharry]

Nothing fancy, but when code is pushed to a branch, CI (continuous integration) should automatically run whatever tests we deem important. This could include:

• alignment of old and new versions of sampler
• convergence of sampler on simple benchmarks
• unit test for correctness of integrator
• a check that the (numerically sensible) momentum update in the code aligns with the mathematical formula from the paper

But need not include anything for now.

I'm going to do this with PyTest, and github's CI.