add tests for GeometricMean

Add tests for GeometricMean, specifically the case without any weights
and the possible panic.
fix spelling mistake
add tests for HarmonicMean

Added tests for harmonic mean, specifically when the weights are nil or
the possible panic.
add test for Hellinger panic

Add test for slice length mismatch.
Add tests to for Histogram panics

Added tests for the possible panics in Histogram.
Add test for JensenShannon panic

Add test when p and q have different lengths.
Reword help for KolmogorovSmirnov

Reworded the first couple sentences to KS, which seemed to be sentence
fragments.