0.2.0 release (#36)

• New functionality:
  • Summary statistics:
    • Harmonic mean
    • Geometric mean
    • Central moments
    • Kurtosis
    • Skewness
  • Information theory:
    • Entropy
    • Cross-entropy
    • Kullback-Leibler divergence
  • Quantiles and order statistics:
    • argmin / argmin_skipnan
    • argmax / argmax_skipnan
    • Optimized bulk quantile computation (quantiles_mut, quantiles_axis_mut)
• Fixes:
  • Reduced occurrences of overflow for interpolate::midpoint
• Improvements / breaking changes:
  • Redesigned error handling across the whole crate, standardising on Result
  • All ndarray-stats' extension traits are now impossible to implement by
    users of the library (see [#34])

Contributors: @jturner314, @LukeMathWalker, @phungleson, @munckymagik