fix: Remove unit C (#2142)

Not sure what this is and why it was there in this place?

**Note**
This contains a very odd fix for a problem I was facing in the integration test which compares the analytical covariance with the numerical one. For some reason the numerical derivative breaks down in the case where we step towards a surface which is 1 cm away with a single step. Using 0.9 cm it works because it will do a second step which seems to provide the necessary accuracy for the differential quotient.
This is not pretty and I would like to take a second look why a single step is not sufficient in this case.

Core/include/Acts/Definitions/Units.hpp

@@ -187,7 +187,6 @@ constexpr double g = 1.0 / 1.782662e-24;
 constexpr double kg = 1.0 / 1.782662e-27;
 // Charge, native unit e (elementary charge)
 constexpr double e = 1.0;
-constexpr double C = J / eV;
 // Magnetic field, native unit GeV/(e*mm)
 constexpr double T = 0.000299792458;  // equivalent to c in appropriate SI units
 constexpr double Gauss = 1e-4 * T;
@@ -240,7 +239,6 @@ ACTS_DEFINE_UNIT_LITERAL(u)
 ACTS_DEFINE_UNIT_LITERAL(g)
 ACTS_DEFINE_UNIT_LITERAL(kg)
 ACTS_DEFINE_UNIT_LITERAL(e)
-ACTS_DEFINE_UNIT_LITERAL(C)
 ACTS_DEFINE_UNIT_LITERAL(T)
 ACTS_DEFINE_UNIT_LITERAL(Gauss)
 ACTS_DEFINE_UNIT_LITERAL(kGauss)
@@ -253,6 +251,8 @@ ACTS_DEFINE_UNIT_LITERAL(mol)
 ///
 /// Unit constants are intentionally not listed.
 namespace PhysicalConstants {
+// Speed of light
+constexpr double c = 1.0;
 /// Reduced Planck constant h/2*pi.
 ///
 /// Computed from CODATA 2018 constants to double precision.

Core/include/Acts/Propagator/detail/GenericDenseEnvironmentExtension.hpp

@@ -146,8 +146,7 @@ struct GenericDenseEnvironmentExtension {
              (stepper.direction(state.stepping) + h * kprev).cross(bField);
       // Evaluate k_i for the time propagation
       auto qopNew = qop[0] + h * Lambdappi[i - 1];
-      Lambdappi[i] = -qopNew * qopNew * qopNew * g * energy[i] /
-                     (q * q * UnitConstants::C * UnitConstants::C);
+      Lambdappi[i] = -qopNew * qopNew * qopNew * g * energy[i] / (q * q);
       tKi[i] = hypot(1, state.options.mass * qopNew);
       kQoP[i] = Lambdappi[i];
     }

Examples/Python/src/Base.cpp

@@ -70,7 +70,6 @@ void addUnits(Context& ctx) {
   UNIT(g)
   UNIT(kg)
   UNIT(e)
-  UNIT(C)
   UNIT(T)
   UNIT(Gauss)
   UNIT(kGauss)

Tests/IntegrationTests/PropagationTests.hpp

@@ -280,7 +280,7 @@ inline std::pair<Acts::BoundTrackParameters, double> transportToSurface(
   options_t<Actions, Aborts> options(geoCtx, magCtx);
   options.direction = Acts::Direction::Forward;
   options.pathLimit = pathLimit;
-  options.maxStepSize = 1_cm;
+  options.maxStepSize = 0.9_cm;
 
   auto result = propagator.propagate(initialParams, targetSurface, options);
   BOOST_CHECK(result.ok());

Tests/UnitTests/Core/Definitions/UnitsTests.cpp

@@ -109,10 +109,6 @@ BOOST_AUTO_TEST_CASE(DecayWidthTime) {
   CHECK_CLOSE_REL(hbar / 1.42_GeV, 4.635295432723526e-10_fs, 1e-7);
 }
 
-BOOST_AUTO_TEST_CASE(Charge) {
-  CHECK_CLOSE_REL(1_C, 6.241509074460763e18_e, eps);
-}
-
 BOOST_AUTO_TEST_CASE(MagneticField) {
   CHECK_CLOSE_REL(10_kGauss, 1_T, eps);
   CHECK_CLOSE_REL(1_kGauss, 1000_Gauss, eps);
@@ -128,10 +124,14 @@ BOOST_AUTO_TEST_CASE(MomentumRadius) {
 }
 
 BOOST_AUTO_TEST_CASE(PhysicalConstants) {
-  using namespace Acts::PhysicalConstants;
+  using Acts::PhysicalConstants::hbar;
   // see https://en.wikipedia.org/wiki/Planck_constant
   CHECK_CLOSE_REL(hbar, 6.62607015e-34 * 1_J * 1_s / (2 * M_PI), 1e-6);
   CHECK_CLOSE_REL(hbar, 4.135667696e-15 * 1_eV * 1_s / (2 * M_PI), 1e-7);
+
+  using Acts::PhysicalConstants::c;
+  // we really want c to be 1
+  BOOST_CHECK(c == 1.0);
 }
 
 BOOST_AUTO_TEST_SUITE_END()