Introduce roles for files related to calculations

[merkys]

During workshop discussions, @giovannipizzi suggested introducing a means to specify how files are related to calculations entries. This PR introduces meta.role field to specify whether an attached file is input or output relative to the calculation in question.

Furthermore, there was a suggestion to order the related output files in a way to make it easier for automated analysis software to parse the output files in order to identify the code and/or calculation type used. Since it is impossible to define the deterministic orders for all the used codes, the suggestion is to use an arbitrary order where output files most likely to contain the identifying information come first. This way analysis software would encounter the identifying output files earlier and stop before reading in all the output files.