DWI::Shells: Changed behaviour RE: shells with few volumes

The previous behaviour of the DWI::Shells class was to discard any shells with fewer than six volumes.
This class will now by default retain all volumes that survive the b-value clustering process, with optional function reject_small_shells() that can be called to remove shells with less than some number of volumes.
The relevant config file option "BValueThreshold" has been changed to "BZeroThreshold".
Closes #63.