add test for KL panic
add test for Mean panic
add test for Mode panic
add tests for Moment

Added test for panic and  weighted case.
reorder special cases in Quantile

Moved the NaN check above the Sorted check, because Sorted is false for
slices with NaN.  This made the NaN code unreachable so I believe it
was unintentional.
add tests for Quantile

Added test fixture for the NaN case, which required a bit of
modification to the test code as well, and added tests for the various
panics that can occur.