Transition to trainstation

Description

Following the release 1.0 of trainstation and prior to release 2.0 of icet, we should transition from the internal fitting module to trainstation. On the icet side, we need to

• remove the fitting module (including tests)
• either * import and "rename" the corresponding parts of trainstation or * change all the imports in test, examples and documentation
• figure out how the references in the documentation should work
• check that all the examples work