MOBT-430: Synthetic data utilities for creating spot-cubes (#1993)

* Synthetic spot data creation utilities added.

Adds the ability to generate spot-data cubes for use in unit tests.
Fixes up calls to the existing gridded synthetic data functions that did not use the keyword arguments explicitly.
Adds unit tests for the new spot-data utilities.
One unit test partially modified as an example.

* Centralise the datetime format definition that corresponds to our typical format.

* Some unit tests that use spot-cubes adapted as a demonstration.

* Fix doc-string indentation issue for sphinx.

* Review response. Improved doc-strings and use of global variables.

* Typo correction.

* Corrects typos highlighted in review.

improver/blending/utilities.py

@@ -51,7 +51,7 @@
     MANDATORY_ATTRIBUTE_DEFAULTS,
     MANDATORY_ATTRIBUTES,
 )
-from improver.metadata.constants.time_types import TIME_COORDS
+from improver.metadata.constants.time_types import DT_FORMAT, TIME_COORDS
 from improver.metadata.forecast_times import add_blend_time, forecast_period_coord
 from improver.utilities.round import round_close
 from improver.utilities.temporal import cycletime_to_number
@@ -334,7 +334,7 @@ def store_record_run_as_coord(
         cycle = datetime.utcfromtimestamp(
             cube.coord("forecast_reference_time").points[0]
         )
-        cycle_str = cycle.strftime("%Y%m%dT%H%MZ")
+        cycle_str = cycle.strftime(DT_FORMAT)
 
         blending_weight = 1
         run_attr = (

improver/metadata/constants/time_types.py

@@ -36,6 +36,8 @@
 
 TimeSpec = namedtuple("TimeSpec", ("calendar", "dtype", "units"))
 
+DT_FORMAT = "%Y%m%dT%H%MZ"
+
 _TIME_REFERENCE_SPEC = TimeSpec(
     calendar="gregorian", dtype=np.int64, units="seconds since 1970-01-01 00:00:00"
 )