Make dim a property of the Set instead of Dat
Make unit tests work after dim migration
Creating a set from HDF5 takes dim as optional argument
Update demos after migrating dim from Dat to Set
Sparsity takes dimensions from the maps
Do not pass dim in unit test Sparsity constructors
Do not pass dim in regression test Sparsity constructors
Update Set docstring
Merge pull request #169 from OP2/multiplicity-on-set

Migrate dim from Dat to Set

The dim i.e. the shape of the data associated with each element of a
Set was previously a property of the Dat. You could declare a
scalar-valued Dat and a vector-valued Dat on the same Set. A function
space (in the FEM sense) was therefore not fully defined by a Set
alone but required information from a Dat defined on that Set. This is
conceptually awkward and is prone to inconsistencies. Therefore we
migrate dim from Dat to Set to make a Set fully define a function
space.