refactor: Allow configurable BField in vertexing examples

[baschlag]

• With the vertexing code previously being templated on the bfield type, a hardcoded ConstantBField was used for simplicity in the Examples. After refactor!: Steppers use MagneticFieldProvider (2/2) #675 went in, a configurable BField can now easily be used and set in the example options.
  This PR changes the Examples accordingly.
• Additionally some clean-up is done and Stepper typedefs like

using Stepper = Acts::EigenStepper<>;

are removed and the type is used explicitly instead.

Closes #735

[codecov]

Codecov Report

Merging #738 (af84b54) into master (12f1247) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #738   +/-   ##
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  Coverage   48.96%   48.96%           
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  Files         325      325           
  Lines       16639    16639           
  Branches     7762     7762           
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  Hits         8147     8147           
  Misses       3042     3042           
  Partials     5450     5450           

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[asalzburger]

Straight-forward adaption to the new MagneticField handling.