Merge branch 'testing'

* testing:
  Update changelog for 3.8 release
  Bump version numbers
  Update from Kasper develop (25cb1351)
  Update from Kasper develop (3fb2d8a6)
  Update from Kasper develop (1f46fd39)
  Update from Kasper develop (fce0771c)
  Update from Kasper develop (37c373cbfa)
  Fix Docker build issue due to putting sources in `/`
  Update from Kasper develop (20f6089daa)
  Update dependency graphs and requirements
  Update from Kasper develop (9d210e7f3c)

Kassiopeia 3.3 documentation

Update changelog

Update to Kassiopeia 3.7.6

* Build fixes (update FindFFTW)
* Add `CurrentTurns` field to KGElectromagnet
* Fix segfault in KGComplexAnnulus

add simple example code

This builds an external application against Kassiopeia libraries.

Update to Kassiopeia 3.7.4

Update to Kassiopeia 3.7.3

kasper-krypton.bash was removed, so no need to load it anymore.

Or, it was merged in PR KATRIN-Experiment#13 but not propagated to the new master.

Update README.md

Update to Kassiopeia 3.7