fix spelling error CumululantKinds

There was a misspelling in the doc for CDF.
add Panics to help test code that should panic

This is from gonum/floats’s tests.  It wraps a called function in a
defer / panic (hopefully) / recover.
Add test for panic in Bhattacharyya
Add tests for panics in CDF

Add test cases for the various ways that CDF can panic.

We might want to have a reserved CumulantKind that is always invalid,
just for testing.
improve bhattacharyyaCoeff short circuit

Old code seemed to be a typo.  There was no difference in result though.
Handle NaN inputs in CDF

When sort.Float64sAreSorted is called, it panics on NaN inputs.

The way the code is written, there seems to be an intent that if the
input x contains a NaN, then the function should return a NaN.  I
switched the order of the conditional statements so that this became
possible.

I also modified the test cases so that cases with NaN could be
evaluated.
test for chi2 special case

Add test for chi squared distance special case, with both bins == 0.
Add test for chi2 panic
remove redundant check for nil

The previous conditional covers the cases where weights == nil, so
there is no need to check the converse.
Add tests for Covariance

There were no tests for covariance, only the example.
simplify implementation of cross entropy

Changed the implementation to match the simpler flow of Entropy.  This
will also result in fewer operations.
Add tests for cross entropy's panic
Add test for ExKurtosis panic

Added a test for the slice length mismatch
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.