Use process "chunks" attribute

Requires arkindex/backend#1766 (closed)

Required for arkindex/backend#1717 (closed)

Instead of relying on a command line argument, use "chunks" from process_information

changed milestone to %Arkindex 1.6.1

added P2 label

assigned to @mlbonhomme

mentioned in merge request !9 (merged)

mentioned in commit b6973ff4

closed with merge request !9 (merged)