Read/Write mmcif/pdbx files

Perhaps this parser would make this easy to support reading writing these files:
[readcif](https://github.com/RBVI/readcif/tree/master)
Sounds like I can just include the header and source file and acknowledge:
"For simplest use, just copy the one C++ header file and source file into your code."
"The readcif library [license](https://github.com/RBVI/readcif/blob/master/LICENSE.md) is an [open source license](https://opensource.org/licenses) with an acknowledgement clause. So if you use it, acknowledge that fact in your documentation."

Here is an example where you extract the parsed info into an atom struct:
https://github.com/RBVI/readcif/blob/master/docs/readcif_example.cpp