MEAM/C: document new reference structures

   Cmin(I,J,K) = Cmin screening parameter when I-J pair is screened

                 by K (I<=J); default = 2.0

   lattce(I,J) = lattice structure of I-J reference structure:

                   dia = diamond (interlaced fcc for alloy)

                   fcc = face centered cubic

                   bcc = body centered cubic

                   hcp = hexagonal close-packed

                   dim = dimer

                   dia = diamond (interlaced fcc for alloy)

                   dia3= diamond structure with primary 1NN and secondary 3NN interation

                   b1  = rock salt (NaCl structure)

                   hcp = hexagonal close-packed

                   c11 = MoSi2 structure

                   l12 = Cu3Au structure (lower case L, followed by 12)

                   b2  = CsCl structure (interpenetrating simple cubic)

                   ch4 = methane-like structure, only for binary system

                   lin = linear structure (180 degree angle)

                   zig = zigzag structure with a uniform angle

                   tri = H2O-like structure that has an angle

   nn2(I,J)    = turn on second-nearest neighbor MEAM formulation for

                 I-J pair (see for example :ref:`(Lee) <Lee>`).

                   0 = second-nearest neighbor formulation off

   zbl(I,J)    = blend the MEAM I-J pair potential with the ZBL potential for small

                 atom separations :ref:`(ZBL) <ZBL>`

                   default = 1

   theta(I,J)  = angle between three atoms in line, zigzag, and trimer reference structures in degrees

                   default = 180

   gsmooth_factor  = factor determining the length of the G-function smoothing

                     region; only significant for ibar=0 or ibar=4.

                         99.0 = short smoothing region, sharp step