Rename femmt.py due to same pip-package name

[gituser789]

Describe the bug
Package problems when having a package name 'femmt' which imports .femmt.py
Seems to be that the package is builds successfully, but problems occour when generating the documentation.
When renaming 'femmt.py' e.g. to 'femmt_classes.py', this works fine.

To Reproduce
Steps to reproduce the behavior:

1. cd sphinx/
2. make clean
3. make html
4. See error

WARNING: autodoc: failed to import class 'MagneticComponent.VirtualWindingWindow' from module 'femmt'; the following exception was raised:
attempted relative import with no known parent package

When opening sphinx/build/html/index.html, the MagneticComponent-Class description is empty.

Expected behavior
Build documentation without mentioned warning, and display a documentation for MagneticComponent-class.