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.
Further clarification of swsh methods, like suggested by Peter Solymos
Merge pull request #63 from jarioksa/swsh-samp-works-with-double

swsh_samp null models can be used with non-integer quantities