rm unused c'tor (and a few trivial tests along)

Sample/Particle/Particle.cpp

Sample/Particle/Particle.h

Tests/Unit/Core/ParticleLayoutTest.cpp

Tests/Unit/Sample/ParticleCompositionTest.cpp

Tests/Unit/Sample/ParticleCoreShellTest.cpp

auto/Wrap/libBornAgainSample.py

auto/Wrap/libBornAgainSample_wrap.cpp