MatrixSpace: 2D (i-j) specialization of VectorSpace

Provides '(i, j)' element access and general forms of inner and outer
products, transpose etc. for square and rectangular VectorSpaces.

VectorSpaces default to be column-vectors as before whereas row-vectors
may be represented as 1xn MatrixSpaces.  In the future it may be
preferable to create a specializations of VectorSpace for column- and
maybe row-vectors but it would add complexity to MatrixSpace to handle
all the type combinations.

Tensor is now a 3x3 specialization of MatrixSpace.

Sub-block const and non-const access is provided via the
'.block<SubTensor, RowStart, ColStart>()' member functions.  Consistent
sub-block access is also provide for VectorSpace so that columns of
MatrixSpaces may be accessed and substituted.

These new classes will be used to create a more extensive set of
primitive vector and tensor types over the next few weeks.

Henry G. Weller
CFD Direct