Differentiate between 'essential' and 'auxiliary' commands
Update description of the 'seeing' command
Update usage output
Update paragraph about the nine commands
Differentiate between 'essential' and 'auxiliary' commands

The nine commands are not always necessary: 'import' is useful if the
FITS images are scattered across multiple directories, 'seeing' when
we need to discard images with bad seeing or elongated, and 'annuli'
to attempt to find the optimal parameters for aperture photometry.
However, simply listing the commands may induce users to think that
all of them must be run sequentially, as we saw in #18. It is better
to differentiate between the essential commands, required in order
to generate a light curve, and those that are only necessary in the
scenarios described above.

This merges branch 'update-lemon-help-message'