<bugfix>Fix issue with subworkflow caching</bugfix> <ticket>#638</ticket>
<bugfix>Fix issue with subworkflow caching</bugfix>
<ticket>#638</ticket>

The bug was due to a caching issue where subworkflows were assuming that the is_cacheable was checked before execution instead of after.  This has been fixed so we can actually check the module's state if necessary as the HTTPFile module does.

core/modules/sub_module.py:
  - Group.compute: keep the persistent_modules locally
  - Group.is_cacheable: check the persistent_modules to see if all can be cached

VisTrails/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,minesense/VisTrails,celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,Nikea/VisTrails,hjanime/VisTrails,minesense/VisTrails,Nikea/VisTrails,celiafish/VisTrails,Nikea/VisTrails,Nikea/VisTrails,hjanime/VisTrails,hjanime/VisTrails,hjanime/VisTrails