Sort out the Workspace data access methods

[NickDraper]

Boost-Python cannot distinguish methds that only differ in overloads be const-ness.
Therefore additional mthods have been added to the workspace class to allow read only access to data from python.

We need to rationalise the methods based on this. Something like
- ReadX, ReadY etc for getting a read access to the data.
- DataX, DataY etc for getting writable access to the data.

Feel free to consider better method names. The methods need to be well documented and consistent changes made to all of the workspaces.

[NickDraper]

This issue was originally trac ticket 179