Coverage is less than 100%

[tomwhite]

The check that coverage must be 100% stopped working at some point. I'm not sure if it's related to the numba jit coverage issue (see #77), but I think we should mark numba functions as no cover (since there doesn't seem to be a way of getting coverage for these functions, see nedbat/coveragepy#849).

We also need to reinstate the mechanism that fails the build if coverage is under 100%, and fix the places not being covered. I'll submit a patch to do all this.