@param fa String with the path to the reference file
@return String with the path to the reference index file, or NULL on failure
If the reference path has the format reference.fa##idx##index.fa.fai,
the index path is taken directly from it as index.fa.fai.
If the reference file is local and the index file cannot be found, it
will be created alongside the reference file.
If the reference file is remote and the index file cannot be found,
the method returns NULL.
The returned string has to be freed by the user at the end of its scope.
@param fa String with the path to the reference file @return String with the path to the reference index file, or NULL on failure
If the reference path has the format reference.fa##idx##index.fa.fai, the index path is taken directly from it as index.fa.fai. If the reference file is local and the index file cannot be found, it will be created alongside the reference file. If the reference file is remote and the index file cannot be found, the method returns NULL.
The returned string has to be freed by the user at the end of its scope.