Fix Constants.h

There seem to be a couple of obvious tasks related to Constants.h:

• it is hidden rather deeply in the namespacing tree, shouldn't we move it up to LOFAR::Constants (which eventually will become everybeam::constants)?
• it is not consistently used throughout, e.g. pi_2 is redefined in https://git.astron.nl/RD/EveryBeam/-/blob/master/hamaker/HamakerElementResponse.cc#L17 and _2pi is used nowhere