Add cross-sample distToNearest mode

[Jason Vander Heiden]

Could add an argument to distToNearest specifying a column containing grouping information which would then only calculate distances across groups (rather than within).

[ssnn]

Started this. Currently works for hs1f & co. I will push when hs5f is ready and I have run some more tests.

[ssnn]

I modified the hs5f part. I have run some tests, but it would be good if you could try to run some more before closing the issue.

[Jason Vander Heiden]

We should probably add an example of doing cross-sample to the vignette.

[ssnn]

Tests pass