OPAL-FEL (MITHRA integration)

Summary

As part of my Master Thesis I am introducing an undulator/wiggler element in OPAL-t. For this reason I have integrated the full-wave solver for undulators MITHRA.

This full-wave solver takes over when the bunch enters the undulator.

• Add to release notes
• Verify coding style
• Update documentation (OPAL/documentation/manual#38 (closed))
• Add Regression Tests (regression-tests#96 (closed))
• Add module and flags to NighltyBuild (regression-tests#99 (closed))
• Resolve MR and all its issues (!361 (merged))