process2d - drop legacy zero bin in size distributions

The legacy zeroth bin for size distributions has finally been dropped from the netCDF files. process2d now supports this The new format takes affect starting with files produced after January 2021 (SPICULE project and forward).

For people generating their own netCDF files. The size distribution variable (e.g. CCDP or C2DC), must have a netCDF attribute:

            CCDP_LWOO:CellSizeNote = "CellSizes are lower bin limits as particle " ;

Older files will not have this attribute or will be listed as "...upper bin limites...".

Refactor Help Menu in xpms2d

Previously the help menu in xpms2d forked firefox directly to resolve help URLs. Now use the OS utilities (open or xdg-open) to open a tab in the default browser.