Implement the handling of schema/table names for SpatiaLiteExposure and PostGISExposure

The addressing of tables is different in SpatiaLite and PostGIS database as we now use schemas in PostGIS. Therefore, we need a function that generates the correct table reference from the table name and potentially a schema name.

\rfc @laurens @gislars @pdlmora @chengzhi @tara