ENH: harmonize matrix constructors (#1220)

- generalize identity matrix constructors for non-scalar types

- add constructors using labelPair for the row/column sizing information.
  For a SquareMatrix, this provides an unambiguous parameter resolution.

- reuse assignment operators

STYLE: adjust matrix comments

src/OpenFOAM/matrices/LUscalarMatrix/LUscalarMatrix.C

@@ -145,7 +145,7 @@ Foam::LUscalarMatrix::LUscalarMatrix
     else
     {
         label nCells = ldum.lduAddr().size();
-        scalarSquareMatrix m(nCells, 0.0);
+        scalarSquareMatrix m(nCells, Zero);
         transfer(m);
         convert(ldum, interfaceCoeffs, interfaces);
     }