YAML: Change the input scheme so that users define input with a YAML file instead of fixed format input

[glwhart]

Snippet of discussion with :
'''
YAML is so established as a format that it is very likely a system would come with a YAML library. I know that libyaml is found on practically every Linux/Unix based system I have encountered, including those on supercomputing systems...I have no idea how easy it is just simply use existing libyaml from a fortran program.

If possible, I would suggest including the Fortran yaml library as part of enumlib, which will automatically be compiled as part of the process and avoid external dependencies. If the open-source fortran-yaml from someone else (e.g., https://github.com/BoldingBruggeman/fortran-yaml) works, that would be best.

It probably would not make sense to adopt YAML if you have to rely on Python (or some other language) to parse it to generate a struct_enum.in file. That is an additional step and software that someone has to install. It is not difficult and in fact, pymatgen essentially generates struct_enum.in from Python objects and it would be trivial for us to support a YAML format (pymatgen already relies on yaml for a lot of things).

'''