Simplify Pr::Tracks

Implements ideas to simplify Pr::Tracks.

Goes with Rec!2239 (merged) Phys!812 (merged)

@olupton @peilian @chasse @sstahl @sponce

• Remove the need for the end-user to write variadic template code
• Provide default (base_class) proxies to reduce boilerplate
• Give more meaning to the Tags allowing to generate index computation and sanity checks automatically, avoiding user mistakes
• Remove the need of preprocessor macros for defining proxies
• Simplify zipping and proxy implementation

Before/after track definition example:

Before:

• https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/TrackEvent/Event/PrDownstreamTracks.h
• https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/TrackEvent/Event/PrTracksTag.h#L123

After:

• https://gitlab.cern.ch/lhcb/LHCb/-/blob/87a56ca04cba1cb956e137dfdd5a471765defeb1/Event/TrackEvent/Event/PrDownstreamTracks.h

New Proxy/Zipping implementation:

• https://gitlab.cern.ch/lhcb/LHCb/-/blob/87a56ca04cba1cb956e137dfdd5a471765defeb1/Event/EventBase/Event/Proxy.h
• https://gitlab.cern.ch/lhcb/LHCb/-/blob/87a56ca04cba1cb956e137dfdd5a471765defeb1/Event/EventBase/Event/SOAZip.h

This zipping/proxy is used by default when requesting an iterator on an SOACollection derived container:

for (auto proxy : tracks.scalar()) {}
for (auto proxy : tracks.simd()) {}

It can also be used as an alternative to the current zipping:

auto zipped = LHCb::v2::Event::make_zip( tracks, muonPIDs );
// instead of
auto zipped = LHCb::Pr::make_zip( tracks, muonPIDs );

• to continue using the current zipping, the macro-style proxy must also be defined
• the new proxy support reading and writing