Check MEAM parameters for t0=1.0, mention this fact in documentation

The alpha, b0, b1, b2, b3, t0, t1, t2, t3 parameters correspond to the

standard MEAM parameters in the literature "(Baskes)"_#Baskes (the b

parameters are the standard beta parameters).  The rozero parameter is

parameters are the standard beta parameters). Note that only parameters

normalized to t0 = 1.0 are supported.  The rozero parameter is

an element-dependent density scaling that weights the reference

background density (see e.g. equation 4.5 in "(Gullet)"_#Gullet) and

is typically 1.0 for single-element systems.  The ibar parameter

    rozero[i] = atof(words[17]);

    ibar[i] = atoi(words[18]);

    if (!iszero(t0[i]-1.0))

      error->all(FLERR,"Unsupported parameter in MEAM potential file");

    nset++;

  }