Standardise NDArray typing

[davidorme]

At the moment, we have rather variable typing of NDArray variables.

We’ve have some variables typed as NDArray[[np.int]] and NDArray[np.str_] but then the demography module has been using NDArray[np.float32] and much of the rest of the codebase has bare NDArray.

The np.float32 in the demography wasn't very well thought through. The rationale is simply to reduce memory requirements and (maybe?) run faster, but because it is so piecemeal it likely isn't effective at either due to type promotion etc. We have #304 to think about addressing this more systematically, but for now we should just type NDArray and do so consistently.