differentiate two ways of suming moments distributions: increase sample or sum up variables.

[lmoffatt]

Two binary operations can be done on a Moment_statistics object:

1) we can combine two samples on the same variable:

  friend  Moment_statistics operator&(const Moment_statistics& one, const Moment_statistics& other) 

or 2) we can add two variables of the same magnitude

 friend  Moment_statistics operator+(const Moment_statistics& one, const Moment_statistics& other)