Fix: Mesh filename handling

[TJMolnar]

• Closes #442

Many URDF files make use of the file:// prefix when specifying filenames for loading geometry or collision meshes. For an example, refer to this line in the xacro for the Kinova Gen3:

<mesh filename="file://$(find kortex_description)/arms/gen3/${dof}dof/meshes/base_link.dae" />

This PR Adds an explicit check for the "file://" prefix in roboticstoolbox/tools/urdf/urdf.py, alongside the existing check for "package://", to handle such filenames appropriately.