'Use nearest resampling for merging temporal coherence (#508)

Closes #144

The `lanczos` filter was leading to pixels above 1 in the merged
temporal coherence image, despite the max being 1.0 per burst.

This is also likely the reason for negative coherences, though
we can reopen if any new instances are spotted (hasn't been seen in a
year)

src/dolphin/workflows/stitching_bursts.py

@@ -132,6 +132,7 @@ def run(
             temp_coh_file_list,
             outfile=stitched_temp_coh_file,
             driver="GTiff",
+            resample_alg="nearest",
             out_bounds=out_bounds,
             out_bounds_epsg=output_options.bounds_epsg,
         )
@@ -157,28 +158,33 @@ def run(
             amp_dispersion_list,
             outfile=stitched_amp_disp_file,
             driver="GTiff",
+            resample_alg="nearest",
             out_bounds=out_bounds,
             out_bounds_epsg=output_options.bounds_epsg,
         )
         repack_raster(stitched_temp_coh_file, keep_bits=10)
 
     stitched_shp_count_file = stitched_ifg_dir / "shp_counts.tif"
-    stitching.merge_images(
-        shp_count_file_list,
-        outfile=stitched_shp_count_file,
-        driver="GTiff",
-        out_bounds=out_bounds,
-        out_bounds_epsg=output_options.bounds_epsg,
-    )
+    if not stitched_shp_count_file.exists():
+        stitching.merge_images(
+            shp_count_file_list,
+            outfile=stitched_shp_count_file,
+            driver="GTiff",
+            resample_alg="nearest",
+            out_bounds=out_bounds,
+            out_bounds_epsg=output_options.bounds_epsg,
+        )
 
     stitched_similarity_file = stitched_ifg_dir / "similarity.tif"
-    stitching.merge_images(
-        similarity_file_list,
-        outfile=stitched_similarity_file,
-        driver="GTiff",
-        out_bounds=out_bounds,
-        out_bounds_epsg=output_options.bounds_epsg,
-    )
+    if not stitched_similarity_file.exists():
+        stitching.merge_images(
+            similarity_file_list,
+            outfile=stitched_similarity_file,
+            driver="GTiff",
+            resample_alg="nearest",
+            out_bounds=out_bounds,
+            out_bounds_epsg=output_options.bounds_epsg,
+        )
 
     if output_options.add_overviews:
         logger.info("Creating overviews for stitched images")