Cothread: Add parameter to control default behaviour of PV.caput

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@121230 e099a375-04f9-0310-9d5f-a741eaff62e1
Cothread: Rework PV object initial value processing

Calling PV.get() on an uninitialised PV object no longer blocks by default.

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@121231 e099a375-04f9-0310-9d5f-a741eaff62e1