More improved variable error reporting

update of COUPLE/simple examples

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

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