Remove --output option
Add output LEMONdB as a positional argument
Replace references to 'options.output_db' with 'output_db_path'
Update usage message
Remove Bash completion for --output
Make the output LEMONdB a positional argument

'diffphot' is the only command that still has the --output option,
using the name 'diffphot.LEMONdB' for the output database unless the
user specifies a different one. Use the same approach as in the other
LEMON commands and require the user to give the output LEMONdB.

This merges branch 'remove-output-option'
Update calls to LEMONdB.get_star()

Fix the ValueError ("too many values to unpack") exception raised by
the juicer GUI when we attempt to open a database: LEMONdB.get_star()
returns an eight-element tuple, but our code expects a five-element
one. This should have been done in commit 4cdc774, but I absolutely
forgot about it. Thanks to Scott Engle for reporting this (#33).