BUG: stats.scoreatpercentile: handle empty input.  Closes gh-1897.
BUG: stats.scoreatpercentile: fix using `per` input as array.  Closes gh-2861
TST: stats.scoreatpercentile: use assert_allclose, clean up imports.

Addresses Josef's review comment.
DOC: stats.scoreatpercentile: mention that np.percentile is preferred.

[ci skip]
Merge pull request #3186 from rgommers/fix-scoreatpercentile

Fix stats.scoreatpercentile