Use marshalow schema to validate synthese get_observations_for_web view

remove use of json_schema
fix #2907

backend/geonature/core/gn_synthese/routes.py

@@ -22,7 +22,6 @@
 from werkzeug.exceptions import Forbidden, NotFound, BadRequest, Conflict
 from werkzeug.datastructures import MultiDict
 from sqlalchemy import distinct, func, desc, asc, select, case
-from sqlalchemy.orm import joinedload, lazyload, selectinload, contains_eager
 from geojson import FeatureCollection, Feature
 import sqlalchemy as sa
 from sqlalchemy.orm import load_only, aliased, Load, with_expression

backend/geonature/tests/test_synthese.py

@@ -29,6 +29,7 @@
 from geonature.core.gn_synthese.models import Synthese, TSources, VSyntheseForWebApp
 from geonature.core.gn_synthese.synthese_config import MANDATORY_COLUMNS
 
+from geonature.core.gn_synthese.schemas import SyntheseSchema
 from geonature.core.gn_permissions.models import PermAction, Permission
 from geonature.core.gn_commons.models.base import TModules