Make euclidean merge available in contrib directory

contrib/dem-blend/README.md

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+# DEM Blending
+
+Blend sets of DEMs by calculating euclidean distance to null values and weighting the combination of elevation models. Based on the split-merge tool within ODM.
+
+Requirements:
+* Directory full of images to blend together
+* NoData should be coded as a value of -9999
+
+## Usage
+
+```BASH
+docker run -ti --rm -v /home/youruser/folder_with_dems:/input --entrypoint /code/contrib/dem-blend/dem-blend.py opendronemap/odm /input
+```

contrib/dem-blend/dem-blend.py

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+#!/usr/bin/env python3
+# Authors: Piero Toffanin, Stephen Mather
+# License: AGPLv3
+
+import os
+import glob
+import sys
+sys.path.insert(0, os.path.join("..", "..", os.path.dirname(__file__)))
+
+import argparse
+import multiprocessing
+from opendm.dem import merge
+
+parser = argparse.ArgumentParser(description='Merge and blend DEMs using OpenDroneMap\'s approach.')
+parser.add_argument('input_dems',
+                type=str,
+                help='Path to input dems (.tif)')
+
+args = parser.parse_args()
+
+if not os.path.exists(args.input_dems):
+    print("%s does not exist" % args.input_dems)
+    exit(1)
+
+output_dem = os.path.join(args.input_dems, 'merged_blended_dem.tif')
+input_dem_path = os.path.join(args.input_dems, '*.tif')
+input_dems = glob.glob(input_dem_path)
+
+merge.euclidean_merge_dems(input_dems
+                    ,output_dem=output_dem
+                )