fix: Nexus_save: Add the measurement entry in the root default path

                  as defined in the standard

heightmap.py

@@ -693,7 +693,7 @@ class HeightMap(object):
         #Create the measurement NXentry as complying to NXmetrology definition
         entry=NXentry(detrended,rawdata,sample,instrument, name='measurement1')
         entry.definition=NXattr('NXmetrology')
-        # add the data group as link to detrended group and set it as default 
+        # add the data group as link to detrended group and set the default path
         entry.data=NXlink(detrended)
         entry.data.set_default()
         # add a the source data to the process group as alink
@@ -713,7 +713,10 @@ class HeightMap(object):
         # erase the old file if any and write the NeXus tree to the file
         if Path(fname).exists():
             Path(fname).unlink()
-        entry.save(fname)
+        #put the measurement entry in the default path
+        root=NXroot(entry)
+        entry.set_default()
+        root.save(fname)
 
 def add_drawing(parent,filename):
     ext=Path(filename).suffix