External potentials in HPMC.

[joaander]

Description

• Implement the infrastructure needed for compiled in external potentials in HPMC.
• Add linear external potential.

Motivation and context

Replace CPPExternalPotential.

How has this been tested?

Added unit tests.

Change log

Added:

* ``hoomd.hpmc.external.External`` provides an abstract interface to external potentials
  (`#1811 <https://github.com/glotzerlab/hoomd-blue/pull/1811>`__).
* ``hoomd.hpmc.external.Linear`` computes linear potential as a function of the distance from a point to a plane
  (`#1811 <https://github.com/glotzerlab/hoomd-blue/pull/1811>`__).
* ``HPMCIntegrator.external_potentials`` sets the list of external potentials applied to the system
  (`#1811 <https://github.com/glotzerlab/hoomd-blue/pull/1811>`__).

Deprecated:

* ``HPMCIntegrator.external_potential`` - use ``HPMCIntegrator.external_potentials``
  (`#1811 <https://github.com/glotzerlab/hoomd-blue/pull/1811>`__).
* ``hoomd.hpmc.external.user.CPPExternalPotential`` - use ``hoomd.hpmc.external.Linear`` or write a custom component in C++
  (`#1811 <https://github.com/glotzerlab/hoomd-blue/pull/1811>`__).

Checklist:

• [x] I have reviewed the Contributor Guidelines.
• [x] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [x] My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.