Mean and variance

[NunoSempere]

• A getMean and getVariance in each module of src/distPlus/distribution/Distributions.re
  • They get the exact answer for the functions in Distributions.re, according to the approximation used.
  • There is now an XYShape.Analysis.integrateContinuousShape function.
• Tests in the tests/Distributions__Test.re function.
  • Calculation of the mean and variance for the normal and lognnormal distributions, at the end.
• I also added some reduce array functions to the E.A. module.

[OAGr]

Nice work. I'll merge then mess with for a bit.