Improve naming of the command-line options

The command-line options are not consistently named and create a bit of confusion. I suggest to change them to:

Required:

• -c, --cell-ids
• -e, --exposure
• -f, --fragilities
• -g, --ground-motion-field
• -p, --polygons
• -t, --taxonomies

Optional:

• -i, --interpolation-method
• -o, --overwrite
• -r, --results
• -x, --exposure-type

\fyi @cnievas @shinde

\approve @fd @ds

\rfc @ds @fd