Fixed a bug for the vector expression x -= y / alpha, where alpha is scalar<T>
Fixed a bug in element_div() introduced by splitting operator tags.

op_div was supplemented by op_element_div, for which the predicate
metafunction was not yet overloaded.
Finished full expression templates for matrices. All tests pass.

Follows the path used for vector expressions.
Some more exotic input combinations may not work yet.
Removed obsolete VECTOR_EXTRACTOR<> and MATRIX_EXTRACTOR<> classes.