Fix: Mesh filename handling (#443)

* fixed handling of mesh filenames in URDF parsing

* minor change to execution flow of filename checks for correctness

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Co-authored-by: Tea Molnar <Tea.Molnar@data61.csiro.au>

Author: TJMolnar

roboticstoolbox/tools/urdf/urdf.py

@@ -302,6 +302,8 @@ def filename(self, value):
 
         if value.startswith("package://"):
             value = value.replace("package://", "")
+        elif value.startswith("file://"):
+            value = value.replace("file://", "")
 
         if _base_path is None:
             value = rtb_path_to_datafile("xacro", value)