Renamed method isHierarchical to nestedCorrelatedConcepts
Added method BP::Model::ColumnType->arrayDimensions, which returns 0
when the column type is not the one from the elements of an array and
more than one when it returns the number of dimensions of this array.
The loading process has now a preparation phase for the data, so
it adapts the data in memory to what it is expected by the storage model.
It is going to be needed by BP::Loader::Mapper::Relational, for instance,
but it will be almost a no-op for BP::Loader::Mapper::MongoDB.
Added bulkPrepare methods to MongoDB and Relational