corrupt session groups can have packet files with multiple APIDs
we handel this by filtering these out and skipping to the next stamp

Because merge was only considering stamp/apid packets for the same
stamp/apid with different byte size were being treated as equal. It now
always treats packets/group with more bytes as more gooder.

Add option to CLI tool to specify VIIRS APID order. **NOT TESTED

Except for the draconian line length limit (used
--max-line-length=99).

* Don't require same file type for all files.
* Fix bugs in packet handling.
* Properly close files to avoid exhausting file descriptors for large
  numbers of RNSCA flies.