Ported partition method (essentially by cut-and-paste with small fixup).
Still filling in the Matrix operations.

1. Factored Matrix map and reduce so that much is shared between seq
and par, using a common argument "parser"
MatrixDecomposeArgsFor[Map/Reduce] and new explicit argument to
MatrixCommon[Map/Reduce] to skip parallel execution path when called
from sequential caller.

2. Cut-and-pasted ParallelArrayFilter to MatrixCommonFilter which uses
a similar strategy to MatrixCommon[Map/Reduce].

3. Added a couple script hints (for call-site cloing) but we probably
need more
Got Matrix.filter working plausibly.  Put in note about problems that
Matrix.pfilter might pose.