Local linear index now passed to Bin / update_task

Instead of passing global indices to the remote update task via Bin<T>,
we now calculate the remote _linear_ (col-major) indices in
ConvergentMatrix::update(), reducing by 1/3 the amount of data to be
copied in by the update task.

This also encapsulates all of the PBLAS distribution config (blocking
factors, LLD, etc ...) in ConvergentMatrix, allowing these to be passed
via template parameters (and potentially substituted at compile-time).