Replace runtime CPPPotential potentials with compile time provided ones.

[joaander]

Description

On trunk-minor:

• Note that CPP potential classes are deprecated.
• Add pair potentials:
  • Lennard Jones
  • Step function
  • Union
  • Angular step
• Add external potentials
  • Linear.
• Add example plugin.
• Update tutorials.
• Update hoomd-validation to use new potentials.

On trunk-major (later, close to the release of v5.0):

• Remove CPP potential classes.
• Port walls and harmonic external potentials to new framework.
• Remove JIT framework.
• Remove LLVM from CMake.
• Remove all mentions of run time compile potentials from the documentation.
• Move all external hpmc potentials to hoomd.hpmc.external
• Port muVT and SDF square well tests.

Motivation and context

The LLVM run time compilation code path is very costly to maintain. Using CPPPotential requires that users write C++ code regardless, so asking them to compile it is not a significant additional burden.