Addition of a function to compute the k-th order central moment of an array#153

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jvdp1 merged 3 commits intofortran-lang:masterfrom

jvdp1:central_moment

Mar 14, 2020

Commits on Mar 1, 2020

Squashed commit of the following:
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Commits on Mar 9, 2020

central_moment: n and mean computed outside select case
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Commits on Mar 12, 2020

central_moment: changed mean to moment in the error messages
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