New feature: STL export

[germa89]

Description of the feature

Currently you cannot export to STL, but you can export the "view" created by the plotting functions using Pyvista.

We should be able to generate STL files of the full geometry without plotting.

Steps for implementing the feature

Reevaluate steps because of comments.

Better implement first DPF

• Generate full volumes, areas, lines, elements, etc. models (not views)
• Create function such as:

mapdl.export("stl", "volumes") 
mapdl.export("stl", "all") 

which will wrap Pyvista Save function.

Useful links and references

• GitHub Discussion:
  STL instead of IGES #1226

[bedermaus]

There is already an operator in DPF-Core which can do it:
https://dpfdocs.pyansys.com/api/ansys.dpf.core.operators.mesh.stl_export.html

[germa89]

Thank you @bedermaus !

[germa89]

If we are going to reuse the DPF operator, we will introduce DPF as a dependency. We should coordinate with #1298