Add feature to l2ss-py to support OB_DAAC lat/lon grid instead of nans for SeaDAS tool

[jamesfwood]

• Needed a specific output to support their SeaDAS tool (https://seadas.gsfc.nasa.gov/)
  • Will probably be a special case that requires an input argument to harmony

[sliu008]

working on it in branch feature/simon_pixel_mask some points brought up by obdaac

1. It's great that you kept the lat and lon in place and nan put the data; Could you please also nan out the l2_flags

2. It's great that you made it a nc file with metadata. Can you make the northernmost_lattitude, southernmost_ lattitude, eastmost_longitude and westmost_longitude reflect the actual value?

3. Is it possible that you make the current way of subset default and add an option that you don't nan out the data and L2_flags (don't cut the corners), for SeaDAS does the subset this way