Function needed to read token file

[LiamPattinson]

Should perform same task as the read_token function in the C++ API:

https://github.com/FAIRDataPipeline/cppDataPipeline/blob/11401582347ab3a11f6a81455393cca20bdee244/src/objects/metadata.cxx#L79-L85

To make sure any future changes are shared across all three languages, it may be a good idea to first add this function to the C API, and then to borrow that in the Fortran API.

[LiamPattinson]

PR raised in cppDataPipeline: FAIRDataPipeline/cppDataPipeline#40