Revision 1802

Added canTag methods to FunctionSpace and AbstractDomain (and its 
offspring).
This checks to see if the domain supports tags for the given type of 
function space.

Constructors for DataTagged now throw exceptions if you attempt to make 
a DataTagged with a FunctionSpace which does not support tags.
To allow the default constructor to work, NullDomain has a single 
functioncode which "supports" tagging.

Fixed a bug in DataTagged::toString and DataTypes::pointToString.

Added FunctionSpace::getListOfTagsSTL.

algorithm(DataTagged, BinaryFunction) in DataAlgorithm now only 
processes tags known to be in use.
This fixes mantis issue #0000186.

Added comment to Data.h intro warning about holding references if the 
underlying DataAbstract changes.

_python_ unit tests have been updated to test TaggedData with invalid 
FunctionSpaces and to give the correct answers to Lsup etc.