refactor: Remove usage of using namespace in headers

[paulgessinger]

We got a bit lenient with not doing

using namespace Acts::UnitLiterals;

in header code. This is a bad idea, because it pollutes the global namespace in every compilation unit such a header is included in. As a workaround, you can put using namespace in any functions where the literals are needed.

This PR removes all of these cases, and also removes a few other instances of using namespace in headers.

[codecov]

Codecov Report

Merging #807 (badade0) into main (31faea5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #807   +/-   ##
=======================================
  Coverage   48.64%   48.64%           
=======================================
  Files         328      328           
  Lines       16900    16900           
  Branches     7935     7935           
=======================================
  Hits         8221     8221           
  Misses       3080     3080           
  Partials     5599     5599           

Impacted Files	Coverage Δ
Core/include/Acts/Propagator/AtlasStepper.hpp	69.36% <ø> (ø)
Core/include/Acts/Propagator/EigenStepper.hpp	67.56% <ø> (ø)
Core/include/Acts/Propagator/Navigator.hpp	57.54% <100.00%> (ø)

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[asalzburger]

Straight forward cleanup.

[asalzburger]

Update and merge afterwards

[asalzburger]

Sorry, forgot to add the explanation in the merge commit.