[math] IllegalStateException thrown by Kurtosis increment and clear methods

[Phil Steitz <ph...@gmail.com>]

Slugging my way through the MATH-854 changes for the stats package,
I have found an anomaly that I would like to fix.  When a Kurtosis
instance is constructed using an external moment, increment and
clear methods do not behave as no-ops as they do for Variance,
SecondMoment and the other moment statistics that can be constructed
using embedded external moments.  Instead these methods throw
MathISE when called on an instance with an embedded moment. 
Changing the methods to behave like the other moment statistics
would technically not be an incompatible change because the
exception is unchecked.  Leaving it as is inconvenient for MATH-854
since we have to then advertise the exception and this is not
consistent with superclass declarations.   Anyone object to this change?

Phil

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