It is still part of the official matrix interface, but I doubt
that anybody actually uses it.  I'd rather see blocklevel gone
and replaced by modern C++ introspection.  Let's remove it here
and see who complains.

[[Imported from SVN: r9587]]

[[Imported from SVN: r9586]]

viz.:

field_type
blocklevel
operator=(scalar)

[[Imported from SVN: r9583]]

[[Imported from SVN: r9582]]