Configurable atom lines in write_input

[tovrstra]

A note from @tovrstra about a feature not supported in PR #188 when writing Gaussian input files: "The format of the atom_lines should be configurable, with about the same level of flexibility as the template for the whole input file. See e.g. the Gaussian Molecule Specification: http://gaussian.com/molspec/. For some job types, more information than element and Cartesian coordinates needs to be added (fragment, charge, ...)."

Originally posted by @FarnazH in https://github.com/theochem/iodata/issues/43#issuecomment-677963867