make Particle class agnostic of underlying energy function

doc/tests/scripts/sidechain_steps.py

modelling/pymod/_reconstruct_sidechains.py

modelling/src/sidechain_env_listener.hh

modelling/src/sidechain_reconstructor.cc

sidechain/doc/rotamer.rst

sidechain/doc/rotamer_constructor.rst

sidechain/pymod/__init__.py

sidechain/pymod/export_particle.cc

sidechain/pymod/export_rotamer_constructor.cc

sidechain/src/CMakeLists.txt

sidechain/src/frame.cc

sidechain/src/frame.hh

sidechain/src/particle.hh

sidechain/src/particle_scoring.cc

sidechain/src/particle_scoring.hh

sidechain/src/particle_scoring_base.hh

sidechain/src/rotamer_constructor.cc

sidechain/src/rotamer_constructor.hh

sidechain/src/rotamer_group.cc

sidechain/src/rotamer_group.hh