Don't force SNP matrices in SNPSample to be of uint8 data type

Instead allow them to be given in any type. We don't enforce or check the type either, for now, so use at own risk.\

As I'd hoped, this was a very simple fix and did not significantly break any existing tests. Just be careful that if you do want to save SNPSamples as uint8 that you make sure to specify the dtype (e.g. especially before writing out files)