Tools for LAMMPS documentation for developers with the "doxygen" documentation platform

More improved variable error reporting

update of COUPLE/simple examples

# Conflicts:
#	src/create_bonds.cpp
#	src/delete_atoms.cpp

Merge branch 'lammps-tools-doxygen' of https://github.com/numericalfreedom/lammps into lammps-tools-doxygen

this now covers a large set of cases where the variable name can be printed.
it also is complete for the current code, since no more default arguments are required

this makes errors for "anonymous" evaluations (if statement, immediate variables) less confusing.

Add ReaxFF Example

CMake compilation fixes

collected documentation fixes

Improved error messages when defining or evaluating variables

Enhance Kokkos Neighbor lists

Kokkos version of Pair SNAP and ZBL

New "coeffs" keyword added to "info" command

are provided.

        New directory: tools/doxygen

        New file:      tools/doxygen/Developer.dox.lammps
        New file:      tools/doxygen/Doxyfile.lammps
        New file:      tools/doxygen/doxygen.sh
        New file:      tools/doxygen/README

The Developer.dox.lammps file contains a slightly revised version of the
Developer.pdf file adopted to the LAMMPS "doxygen" documentation.

The Doxyfile.lammps file is a first proposal for a LAMMPS "doxygen"
documentation flavor and can be adjusted to specific requirements.

The "doxygen.sh" shell script generates the LAMMPS "doxygen"
documentation.

Detailed instructions can be found in the README file.