rename Claim._formatDataValue() into _formatValue()

and create a new _formatDataValue() method
which returns _formatValue() as 'value' and the value type as 'type'
(technically, this is a breaking change, but at internal level)

for clarity reasons, and also to reduce code duplication in site.py
and in Ibe77c51fc5cf3dae8ece533cc5787bf960f0ebed (yet to come)

this allows claims of "commonsMedia", "globe-coordinate" and "quantity"
datatypes to be added as sources using DataSite.editSource()

Change-Id: If8de1887f34d83a38e1a0b05279c5b8eb68a1e8a
Merge "rename Claim._formatDataValue() into _formatValue()"