make importing openquake optional

nzshm_model/psha_adapter/openquake/hazard_config.py

@@ -13,12 +13,18 @@
 import pathlib
 from typing import Dict, List, TextIO, Union
 
-from openquake.hazardlib.site import calculate_z1pt0, calculate_z2pt5
-
 from .hazard_config_compat import check_invariants, compatible_hash_digest
 
 log = logging.getLogger(__name__)
 
+try:
+    from openquake.hazardlib.site import calculate_z1pt0, calculate_z2pt5
+except ImportError:
+    log.warning(
+        """warning openquake module dependency not available, maybe you want to install
+                with nzshm-model[openquake]"""
+    )
+
 
 class OpenquakeConfig:
     """Helper class to manage openquake configuration files.