Reduce vertexing template arguments and introduce linearizer

In order to be able to use different track linearizer for vertex fitters in the future (see #634), the vertex fitters need to accept different linearizers. Currently only one linearizer based on a helical track model (LinearizedTrackFactory) is implemented. This MR:

• renames the current linearizer LinearizedTrackFactory -> HelicalTrackLinearizer
• introduces the template parameter linearizer_t for vertex fitters together with a linearizer concept
• removes bfield_t template argument from vertex fitter
• removes the propagator_t template argument from the vertex fitter since this type can be deduced from the linearizer_t

In addition:

• All template parameters except for vfitter_t are removed from the vertex finders since all previously templated types can now be deduced from the fitter
• Introduces seed finder template argument for vertex finder

Change of ownership:

• Propagator shared_ptr (and for current linearizer also BField) now held by linearizer itself
• Finder holds linearizer and passes to fitter in fit() method

Additionally: Updates IterativeVertexFinder by replacing naked for loop by std::find_if