swsh_samp null models can be used with non-integer quantities

`"swsh_samp"`, `"swsh_samp_c"` and `"swsh_samp_r"` null models shuffle original
cell values and can also be used with non-integer data. They were
unnecessarily restricted to integers. The `"swsh_both"` models
redistribute individuals among non-zero cells and can only be used
with integer values, and they are not changed in this PR.

@psolymos : will you want to have a look at this? All this does is to relax the integer requirement in these three null models (we needed this in one project).