New capabilities for updating KIM portable models parameters

1. Add new capabilities within LAMMPS for setting parameters of
the KIM portable models (PM). The current implementation extends
the 'pair_coeff' command to modify the PM parameters.
A series of additional parameters after the species list is the
input to the modified command. Each set of the other setting
is parameter name, range of array indices, and corresponding
parameter values.

2. Add two getters to the PairKIM class to get the KIM_Model object,
and the atom type list outside the 'pair_kim' command for ease of
use and extension ability of a new 'kim_param' command.