Doc and example fixes

update docs before patch release

convert bigint values for bonds/angles/dihedrals/impropers to doubles

Add python pair style for implementing simple pairwise additive potentials in python

convert bigint values for bonds/angles/dihedrals/impropers to doubles when evaluating those keywords in variable expressions

port thread-safe temperature biasing from LAMMPS-ICMS

Import multi-element compatible pair style edip as edip/multi

Bugfix for correct resetting of previously set cutoffs to various Pair::settings() functions

Add Gao-Weber manybody styles

AIREBO spline code out-of-bounds and bondorder derivative bugfix and refactor

Collected small updates and bugfixes

corrections to pair style morse/smooth/linear

bugfix for 2 recenty reported neighbor issues, also a ReaxFF fix species update from Stan

lock the sphinx command to version 1.5.6, since version 1.6.x seems to break one of the extensions we use

- make all python potential classes derived from LAMMPSPairPotential
  which contains shared functionality. We currently don't check
  for supported atom types. may want to add that again later.
- keep track of skipped atom types in the C++ code.
- add test against units setting. must set self.units='...' in constructor
- make compute_force method consistent with Pair::single() in LAMMPS and return force/r instead of force.
- rename potentials.py to py_pot.py
- update test runs. some small tweaks.

with the single function, python pair styles can be massively
sped up and made compatible to accelerators, as one can translate
the analytic force and energy functions through LAMMPS into suitable
tables and then simply use the on-the-fly tables for production runs

Implements a class loader which takes a fully qualified Python class
name, loads the module and creates an object instance.

To add flexibility, the current working directory and the
directory specified by the LAMMPS_POTENTIALS environment variable are
added to the module search path.