StepActor

This MR considers the propagation in a dense environment. It uses the possibility to build a TrackingVolume out of material. In order to perform the propagation through the material, the function that is integrated by the RKN4 is adapted to involve the energy loss contributions by ionisation, radiation, pair production and photonuclear interaction. The corresponding formulas are also provided by this MR. This MR provides an generalization of the EigenStepper by using a collection of different environments that can be user defined from whose the stepper chooses the best fitting environment in each step. The decision is based on the Auctioneer concept which will be added by this MR. For testing the propagation of the covariance matrix in the dense environment, a CuboidVolumeBuilder is implemented. This builder is a simple builder for cuboid shaped detectors. This implementation does not demand to be a complete building factory but just to allow an easy builder for detectors for unit tests.

Closes ACTS-534, ACTS-484 and ACTS-444