Option to specify catalog type in more places

[cholmes]

This is probably a useful thing for other 'convert' commands, but this is the one I'm using the most.

Most of my workflows involve eventually sending the catalog out, either uploading to google cloud or putting into a zip to send someone. It'd be nice if I could just continually work with self-contained catalogs, instead of having to remember to do a copy with -t. This hits me most with planet convert, but also probably in merging (or my guess is it might).