Add support for empty matrices and properties
Changes required for the Gentoo platform
Change submatrix specification to use corners

Introduced a new data type (IJ) to designate coordinates within a
matrix. Switched all routines that need submatrix coordinates
accordingly, keeping the old interfaces available as overloads.
Merge branch 'master' of github.com:MauroCalderara/LinAlg

Conflicts:
	include/types.h