Src info to output

[pdeiml]

This PR adds all basic source info files which are stored in $GAMMACAT/input/sources to the output collection, more precisely, to $GAMMACAT/docs/data/sources.
See comment of @cdeil here:
#177 (comment)

Ready to merge (but look at the inline comment!)

[cdeil]

This looks good, except for one issue: the reference_id isn't filled with something useful, it's a string serialisation of a Python list:

    {
        "source_id": 3,
        "reference_id": "['2017ApJ...836...23A', '2011ApJ...730L..20A']",
        "file_id": -1,
        "type": "bsi",
        "location": "sources/tev-000003.yaml"
    },
    {
        "source_id": 4,
        "reference_id": "['2012AIPC.1505..490B']",
        "file_id": -1,
        "type": "bsi",
        "location": "sources/tev-000004.yaml"
    },

Fixed in 1b07ab6.