fix test_public_headers_build test target

• add missing include in public header
• delete (seemingly) unused header ← this is why I wip'ed as I believe this is potentially harmful
• explicitly state include dependency in cmake lists file (w/o default build target)

The idea is that if one has a cpp file with only one line #include "public_header.h", where public_header.h is a file that we install in InstallArea/…/include, then that file should compile without errors and warnings. Ideally that helps when sorting header files alphabetically.

@jonrob: as you can see in the changes, I deleted a Rich header which I didn't see used anywhere (in my checkout of projects until Moore and with lb-glimpse until DaVinci). istm like LookupTableInterpolator.h wasn't included anywhere, but I believe it's still good if somebody who knows about RichUtiles can confirm it's obsolete.

@clemenci: the edition of the cmake lists file shouldn't do anything visible in a standard build but maybe it's good to have the include dependency known when migrating to the new configuration?

@ibelyaev: I edited TreeHelpers.h because it uses Decays::Node m_head = Decays::Nodes::Invalid, which means having Decays::Nodes::Invalid defined is a good idea. This isn't an issue when building LHCb as all files which included TreeHelpers.h seem to have included Kernel/Nodes.h (through NodesPIDs.h in the case I looked at) as well.