Small fix for underway ST instrument (#257)

* add small buffer to spacetime region, may help avoid some issues

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: j-atkins

src/virtualship/instruments/ship_underwater_st.py

@@ -58,14 +58,18 @@ class Underwater_STInstrument(Instrument):
     def __init__(self, expedition, from_data):
         """Initialize Underwater_STInstrument."""
         variables = {"S": "so", "T": "thetao"}
+        spacetime_buffer_size = {
+            "latlon": 0.25,  # [degrees]
+            "time": 0.0,  # [days]
+        }
 
         super().__init__(
             expedition,
             variables,
             add_bathymetry=False,
             allow_time_extrapolation=True,
             verbose_progress=False,
-            spacetime_buffer_size=None,
+            spacetime_buffer_size=spacetime_buffer_size,
             limit_spec=None,
             from_data=from_data,
         )