Don't raise gemmi ModuleNotFoundError unless gemmi is used (dials#2592

)

* Hide gemmi imports so `ImportError` is not raised unless gemmi is used

* News fragment

* Removed whitespace introduced by accident

* ... and add missing whitespace

* Make gemmi import "Up-front but nullable", re-raise if needed

* Make is specific: `ModuleNotFoundError`

newsfragments/2592.bugfix

@@ -0,0 +1 @@
+Reorganize imports in ``export_mtz.py`` so that its gemmi-less objects can be used by other gemmi-less software

src/dials/util/export_mtz.py

@@ -8,9 +8,13 @@
 from math import isclose
 from typing import List, Optional
 
-import gemmi
 import numpy as np
 import pandas as pd
+try:
+    import gemmi
+except ModuleNotFoundError as e:
+    gemmi = None
+    gemmi_import_error = e
 
 from cctbx import uctbx
 from dxtbx import flumpy
@@ -310,6 +314,8 @@ def add_batch_list(
     if max_batch_number > batch_offset:
         batch_offset = max_batch_number
 
+    if gemmi is None:
+        raise gemmi_import_error
     batch = gemmi.Mtz.Batch()
 
     # Setting fields that are the same for all batches
@@ -670,6 +676,8 @@ def export_mtz(
         )
 
     # Create the mtz file
+    if gemmi is None:
+        raise gemmi_import_error
     mtz = gemmi.Mtz(with_base=True)
     mtz.title = f"From {env.dispatcher_name}"
     date_str = time.strftime("%Y-%m-%d at %H:%M:%S %Z")