Add more ways to specify integration grid and BZ sampling

[davidbowler]

It would be useful to be able to specify the integration grid in terms of a grid spacing required, and to specify BZ integration in terms of a spacing in reciprocal space (these are both easy to do!).

It would also be useful to remove the restriction on numbers of grid points which were imposed by using GPFA (multiple of 2, 3, 5, 7 only) and possibly to introduce flexibility in terms of block size to make grid spacing easier to fit.