Introduce GroupingConfiguration concept tree

The get_grouping_information method in PropertyStatistics
is doing way too much, and relies on hairy conditionals
(especially since adding the year grouping in 0a8e7fb).

Here we introduce a new concept, GroupingConfiguration:
AbstractGroupingConfiguration
└── PropertyGroupingConfiguration

├── ItemGroupingConfiguration
└── YearGroupingConfiguration

The construction of the SPARQL query for get_grouping_information
is now handled by the get_grouping_information_query method
in the GroupingConfiguration, using polymorphism to handle
the grouping by item and grouping by Year.

This reflects the Line concept tree defined in c18611b63.