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
  • Gravity.
• 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.

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.