Expose ACTS unit constants to python

As discussed in !47319 (merged), we want to make the ACTS unit constants available to the python configuration, and make sure they're in sync.

This MR adds a dictionary that is generated from the Acts/Definitions/Units.hpp file and a wrapper python module that wraps it to hide the struct trick needed to autoload the variables (this workaround is borrowed from xAODEgamma/EgammaParameters. I also added a test that checks the current units in the ACTS header are available in python, and that they are non-zero.

/cc @akraszna @fwinkl @cvarni

Tracking/Acts/ActsInterop/ActsInterop/ActsUnitDict.h

+// Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
+
+#ifndef ACTSINTEROP_ACTSUNITDICT_H
+#define ACTSINTEROP_ACTSUNITDICT_H
+
+#include "Acts/Definitions/Units.hpp"
+
+namespace ActsUnitConstants {
+  struct ROOT6_NamespaceAutoloadHook {};
+}
+
+
+#endif