Time-dependent Link-based Ranking Method

This ranking method combines the link-based ranking with the time-dependent
citation counts. This ranking method is not very well suited for data
collections that allow cycles, since for strong time decay parameters,
it tends to overestimate the value of recent publications.

Algorithm:
    1.  Read the citation data from the database or from a file
        that has the following format: x[tab]y where the paper with
        recid x cites the paper with recid y. The citation data is
        stored in a map key:value, where the 'key' is a record id
        and the 'value' is the list of publications that cite
        publication 'key'.

    2.  Read the publication dates for each paper from the
        database or from a file that has the following format: x[tab]y,
        where x is a recid and y is the publication year. There are
        several possibilities for retrieving the dates from the database:
        i) using the 260__c MARC tag that contains only the publication
        year (this is the option that we are using);
        ii) using the 269__c MARC tag that contains the complete
        publication date.
        For the papers that do not have a publication date, we consider
        the date of insertion in the database (961__x tag). If neither
        the publication date nor the insertion date are available,
        we consider an average date (computed with the existing dates).

    3.  Read the convergence threshold, check_point and damping_factor
        parameters from the configuration file. (parameters specific
        for the link-based ranking)

    4.  Read the time_decay factor from the configuration file.

    5.  Remove the simple loops (loops caused by  x cites y and
        y cites x, and loops caused by x published in year t1 cites
        y published in year t2, but t1 < t2).

    6.  Calculate the weight of each citation based on its age
        and time_decay.

    7.  Calculate the final rank for each publication.

    8.  Write the ranks to the database and to a file. The name of
        the file and the number of ranks that should be outputted can
        be set into the configuration file. If the name of the file
        is not set, then the ranks are only written in the database.