Reorganize into higher level module folders

Closes #594 (closed)

Here's the proposed new organisation for the Core modules:

Module	Content	Comment
Surfaces	all top level surface classes and surface bounds	separated from rest of geometry because of EDM bindings
Geometry	all other geometry calls and geometry building tools	GeometryID, GeometryContext moved from Utilities
EventData	all event data objects
Material	all material classes and material mapping tools	MaterialInteractor to stay in Propagation
MagneticField	all magnetic field related classes	MagneticFieldContext, MagFieldUtils from Utilities
Propagator	all propagation and extrapolation classes	including Navigator etc
Fitter	all fitting classes	KalmanFitter, GSF, etc.
Seeding	everything with seeding	might need a renaming to pattern recognition ?
Vertexing	everything with and for vertexing
Utilities	the original utilities container

The exact same modules should then also appear on the webpage (I have started working on this) as Core modules and in the Tests container.

Then, plugins should indicate some external dependency, e.g.

• TGeoPlugin: binding to TGeo geometry
• DD4hepPlugin: binding to DD4h4p geometry
• GeoModelPlugin: binding to GeoModel geometry
• RootPlugin: binding to root readers/writers
• JsonPlugin: json based reading/writing

In a follow-up MR, the following Plugins will disappear:

• DigitizationPlugin: code will split into acts-core/Core for the SpacePoint building and acts-fatras for the digitisation
• IdentifiacationPlugin: not needed anymore after DigitizationPlugin goes