Changelog: version 0.2.0

diff since 0.1.6

This release is part of the restructuring processes of QED.jl (see QEDjl-project/QuantumElectrodynamics.jl#35 for details).
It is a breaking release, indicated by the bumped minor version because we don't have a major version for this
yet.

Breaking changes

This release removes the core functionality and moves it to QEDcore.jl.
The purpose of this package is to transform from the toolbox status to the provider of all
common interfaces.

New features

This version introduces the interfaces used downstream in QEDcore.jl and
QEDprocesses.jl. Among the interfaces already
present in QEDbase.jl, we add

• the process interface from QEDprocesses.jl
• the model interface from QEDprocesses.jl
• the particle stateful interface
• the phase space point interface
• the differential probability and cross-section interface from QEDprocesses.jl

See #68 for details.

Maintenance

Besides the new interfaces, this release contains some maintenance and minor changes and
fixes

• pretty printing for particles and spin/polarization #65 #61
• scalar broadcasting for particles, directions, spins, and polarizations #62
• spin-/polarization multiplicity convenience functions #63
• add is_incoming and is_outgoing to the exports #60
• add AbstractFourMomentum to the exports #66
• fix of bispinor mul #64