Removed execution of svd_clean when importing it

If svd_clean.py was imported it crashed since the global space executed the script.
Added starter and main function.
Need to be importable for doc creation with sphinx.
Removed globals in svd_clean and adapted docstrings

Put all globals from svd_clean into new static class _InputData.
Adapted docstrings in several files and extended sphinx documentation with
svd_clean.py module.