New constructor for LocalMatrix + some cleanup

New constructor takes _ld as a parameter, so that as_local_matrix() can
return a LocalMatrix with _ld == LLD /= _m
Added support for regular progress() calls

Now after every _progress_interval calls to ConvergentMatrix::flush(),
the thread will pause to clear its local task queue via
upcxx::progress()
Namespace change ("convergent" -> "cm")