Improve XML tags for Function data io

From the mailing list:

>> After a long absence, Function i/o to file has made a come back. It
>> was particularly necessary to support running code in parallel that
>> reads in data for Functions (such as the advection-diffusion demo). We
>> have more demos running in parallel now, and it's now possible to
>> restart parallel computations reliably (and even on a different number
>> of processes).
>>
>> When sending a Function to an XML file, there is an entry for each
>> vector (dof) entry, e.g.
>>
>> <dof global_index="2" value="0" cell_index="3" local_dof_index="0" />
>>
>> 'global_index' is the vector index, 'value' is the entry value,
>> 'cell_index' is a cell that the dof belongs to and 'local_dof_index'
>> is the local dof number relative to the given cell. With this
>> information, it's possible to read in and use function data
>> independently of the new dof map. If anyone has suggestions or
>> objections to this format, speak up now.
>
> Is the cell index global to the mesh?
>

Yes.

> For consistency, I'd suggest the following names:
>
> <dof index="2" value="0" cell_index="3" cell_dof_index="0" />
>

OK.

Garth