Discrete element models for the BODY package

OpenCL related fixes and improvements to the GPU package

Fixes a memory leak when using OpenCL

- document previously undocumented OpenCL tune settings
- implement OpenCL platform selection through prefixing the device type with the platform id separated by a colon
- allow passing custom tune parameters though postfixing the device type with the 13 tuneable parameters separated by commas
- remove an extra clear() that would delete device properties structs an cause LAMMPS to output garbage strings

USER-DPD: propagate a minor performance bugfix throughout the DPDE code

The GPU package uses OpenCL events for measuring time. These have to be
released to free up memory. I removed the clReleaseEvent() calls in the
clear() method because in some cases they don't exist yet and I couldn't
find a way to check for a valid event (clRetainEvent didn't work). This
at least fixes the massive leak during simulations.

See issue #1006

The fix_shardlow_kokkos.cpp code had already factored out a redundant
sqrt() calculation in the innermost loop of ssa_update_dpde().  This
changeset propagates an equivilent optimization to:
  fix_shardlow.cpp
  pair_dpd_fdt_energy.cpp
  pair_dpd_fdt_energy_kokkos.cpp
The alpha_ij variable was really just an [itype][jtype] lookup parameter,
replacing a sqrt() and two multiplies per interacting particle pair
by a cached memory read.  Even if there isn't much time savings, the
code is now consistent across the various versions.

+ added examples for special cases with disks and rods for 2d, and spheres and rods for 3d,
+ corrected the definition of the cutoff distance in pair style command

include support for 'extra/XXX/per/atom' info in restart files

Allow 'set' command to change atom velocities

Fix rare crash in Kokkos ReaxFF on GPUs

docs: Fix sneaky unicode character

cmake/README.md: fix GPU_ARCH options

Port enforce2d kokkos

Fixes the `pdf` target of the `Makefile`.

2 small bug fixes to load balancing

better rRESPA doc page, also a new Makefile.theta

Collected small updates for the next patch release