elements: added function to convert between index and atom char

It's very inefficient as of now. I'll improve it later. Point is, I'd rather not store 100000 arrays and use the chars as index. It's impractical and highly unnecessary. I'll make a struct in elements.h with every atom's attributes (charge, mass, color(for OGL's drawings), etc.).
parser, toxyz, molreader: use atomic index instead of char, make use of elements f-n

What was I on when calculating bonds in molreader. After 0.1 I'll want to make use of Lua or something instead.