[feature request]: add option -f to parse one file only

[wkliao]

We often see applications accessing more than a file. In stead of parsing all files
involved in an application run, this feature request allows to focus on one
file of interest. Here is an example usage.

darshan-dxt-parser \
    -f /scratch/wkliao/canonical_I_out_h0.nc \
    /log/path/wkliao_e3sm_io_id1022-1022_2-11-52977-6878012346360337959_1.darshan

A similar feature request can also be proposed for darshan-parser.