Add convert-to-pds subcommand

[kmgill]

Add a subcommand which will convert a calibrated image to PDS format. If this requires writing a whole VICAR support library, then lower the priority due to larger scope of work.

[kmgill]

Likewise, if a VICAR library is written, then create a pds-to-png subcommand.

[sschmaus]

perhaps this functionality could be implemented via gdal

[kmgill]

Yeah, it can be done by gdal as it understands VICAR. Not as fun/interesting as writing my own implementation too. Vicar is pretty straightforward, though writing a serializer/deserializer (I'm researching using serde for this) for the PDS v<=3 PVL labels is a bit more complicated.