refactoring dmat.  iterators not in yet, so matrix to-from mutablematrix is NOT functional
placed changes to EngineTests.m2 from master branch in linalg-refactor version, and removed some header file includes
Working with Jakob: added dmat iterators, rewrote and reworked MutableMatrix::to_matrix
Changed most uses of DMat::array() to use iterators.

Still using array(): ffpack routines, dmat-LU.
A number of issues fixed.

ZZ * MutableMatrix, complex number internal functions now use const,
engine rings: two bugs found were fixed.