enum listwise vs. pariwise

[Jason Stover]

Right now, LISTWISE and PAIRWISE are enum constants in
covariance-matrix.h.

Pairwise vs. listwise deletion will be a consideration for many other
procedures. John suggested putting them in a common location, probably
missing-values.h.

So what does everyone think of adding the following to
missing-values.h?

enum mv_handling
{
        MV_PAIRWISE,
        MV_LISTWISE
};

-Jason