Allow extracting data with repeated coordinate descriptors

[sandorkertesz]

Is your feature request related to a problem? Please describe.

Related to #51

The data in question has the following structure:

And we would like to perform:

df = pdbufr.read_bufr(f, 
    columns=("pressure", "pressure", "u", "v"),
)

to get results like this:

49660,55520,2.2,-0.9
49660,55520,0.3,0.4

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