Add ability to create PartialRowset updates

To validate: syn.store() a PartialRowSet. It is easier to use the classmethod PartialRowSet.from_mapping(). An example is provided in the documentation of the PartialRowSet class

To validate, please try to use the PartialRowSet class to create a partial change to a table. There is a classmethod PartialRowSet .from_mapping() that should make this easier to use. An example is in the documentation of the PartialRowSet class (not the from_mapping() classmethod)

So I want to point out that this change doesn't exist on the develop branch. It went straight into master.