s/cpd/csd/ for Optech reader
s/cpd/csd/ for Optech reader

I fat-fingered the file extension for Optech files back when I made the
plugin, and only now discovered the error. Optech files are `.csd`, not
`.cpd`.

lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL