Add support for external application schema mapping tools

[tfr42]

Currently deegree supports it's own XML mapping configuration file to map a GML application to an E/R model.

This feature request proposes that deegree supports also external tools such as:

• shapechange - SQL DDL output
• gdal GMLAS using OGR layers and fields (requires gdal 2.2+)
  to generate the SQL DDL and if possible to build a FeatureStore on the configuration files used by those external tools.

For image raster data deegree provides already a GdalTileStore using gdal as a datasource provider.

[tfr42]

Documentation how gdal GMLAS driver derives relational mapping from GML application schemas is available at https://github.com/BRGM/gml_application_schema_toolbox.