Update class40 example

[greschd]

The class40 example needs to be updated to make it more approachable:

• The input file should be stored in example_data and retrieved via helper methods.
• In the header of the class40 example, it is noted that MAPDL + DPF are run in docker containers. We should either:
  • remove the note if the code works as-is also in a local context
  • explain how to run the example (or fix it s.t. it runs out-of-the-box) with a unified installer