Allow copying of results from read-only workflow directories (like eMan shared sandboxes)

[jhclark]

Based on conversation with @ales-t, add a feature that allows sharing trusted repositories of intermediate results.

E.g. If Jon and Ales are both working on a WMT system using the same code repository, they can both have their own output workflow directories. However, partial results should still be able to be copied from the other user's directory (along with the software version and other workflow history).

Keeping things read-only still prevents difficult ownership planning and eliminates the need for cross-user locking, etc.