Cothread: Add sync() method to PV_array

sync() will block until a value is retrieved for each PV.  Also
return copies of numpy arrays so that values do not change
unexpectedly.

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@108793 e099a375-04f9-0310-9d5f-a741eaff62e1
Cothread: Refinement to PV_array .caget method and update documentation

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@108794 e099a375-04f9-0310-9d5f-a741eaff62e1