Iov improvements

[pikacic]

Trying to use dd4hep::IOV we found a things that could be improved:

• use modern C++ instead of legacy C
• IOV::Key was allowing for different types for the two bounds of the interval, but the feature was not used and could lead to weird situations
• the value type in the IOV::Key was long, but that's not portable across architectures, so I changed it to std::int64_t

BEGINRELEASENOTES

• Modernization and clean up of dd4hep::IOV (using instead of typedef, constexpr instead of enum)
• Make dd4hep::IOV::Key uniform wrt first and second type
• Use std::int64_tinstead of long for dd4hep::IOV::Key elements

ENDRELEASENOTES

[andresailer]

There are a number of tests failing:

The following tests FAILED:
	  4 - t_AlignDet_Telescope_dump_xml (Failed)
	  5 - t_AlignDet_Telescope_write_xml (Failed)
	  6 - t_AlignDet_Telescope_stress (Failed)
	  9 - t_AlignDet_Telescope_readback_xml (Failed)
	 10 - t_AlignDet_CLICSiD_stress_LONGTEST (Failed)
	168 - t_Conditions_Telescope_root_load_usr (Failed)
        175 - t_Conditions_CLICSiD_root_load_usr_LONGTEST (Failed)