Error when trying to save rotor with coupling element

[jguarato]

As mentioned in #1134, an error was encountered when saving a rotor with coupling element:

File d:\ProgramData\Anaconda3\Lib\site-packages\spyder_kernels\py3compat.py:356 in compat_exec
exec(code, globals, locals) File ..\wvc_insideshafting\ross10v01_inside_coupling.py:107
rotor.save('rotor.toml')
File d:\ProgramData\Anaconda3\Lib\site-packages\ross\rotor_assembly.py:3132 in save
el.save(file)
File d:\ProgramData\Anaconda3\Lib\site-packages\ross\shaft_element.py:411 in save
"name": self.material.name,
AttributeError: 'CouplingElement' object has no attribute 'material'