add calculation and plotting of discrete mean and moving mean (running average)

[TatianaBurek]

Request from Binbin - December 2, 2019

Given a series of numbers and a fixed subset size, the first element of the moving average is obtained by taking the average of the initial fixed subset of the number series. Then the subset is modified by "shifting forward"; that is, excluding the first number of the series and including the next value in the subset.
discrete mean - the average of the fixed subset of the n-days or n-month

numpy.convolve function can be used for both(?) cases

[TatianaBurek]

Helpful links:
https://stackoverflow.com/questions/13728392/moving-average-or-running-mean