MBExpr preliinarily layed out.

The main objects in this package, MBExpr, many-body expression, has been
set. It is designed to represent a sum of sums of primitive products of
creation and annihilation operators. Now the basic structure of this
class and the change of basis has been preliminarily finished. What is
pending is the implementation of addition, scalar mulitplication, and
the multiplication operation for them.