ch4: methane-like structure only for binary system.
dia3: diamond structure with primary 1NN and secondary 3NN inteation
tri: H2O-like structure that has an angle
zig: zigzag structure with a uniform angle
lin: linear structure (180 degree angle)
** tri, zig, and lin reference structures require angle information (in degree)
such as the following.
theta = 109.5
- use static/const
- return instead of ptr-parameter, &ref if more than one return
- replace macros from header with inline functions
- remove useless/old comments
