documentation update for MEAM to clarify the I,J,K indices in the MEAM parameter file

keyword(I,J) = value

keyword(I,J,K) = value :pre

The recognized keywords are as follows:

The indices I, J, K correspond to the elements selected from the

MEAM library file numbered in the order of how those elements were

selected starting from 1. Thus for the example given below

pair_coeff * * library.meam Si C sic.meam Si Si Si C :pre

an index of 1 would refer to Si and an index of 2 to C. The recognized

keywords for the parameter file are as follows:

Ec, alpha, rho0, delta, lattce, attrac, repuls, nn2, Cmin, Cmax, rc, delr,

augt1, gsmooth_factor, re