Recognize @unit parameter documentation tag. Not used in any output yet.

Tools/px4params/srcparser.py

@@ -44,6 +44,7 @@ class Parameter(object):
         "default": 6,
         "min": 5,
         "max": 4,
+        "unit": 3,
         # all others == 0 (sorted alphabetically)
     }
 
@@ -86,7 +87,7 @@ class Parser(object):
     re_is_a_number = re.compile(r'^-?[0-9\.]')
     re_remove_dots = re.compile(r'\.+$')
 
-    valid_tags = set(["min", "max", "group"])
+    valid_tags = set(["group", "min", "max", "unit"])
 
     # Order of parameter groups
     priority = {