(this and following issues contributed as part of JOSS review)

Overall looks pretty great. Just a couple of little fixes.

• Level-0: "corresponding" and "solutions, space"
• Level-1: want to mention the laser ranging data for GRACE-FO? Might also want to mention the auxiliary Level-1b data produced by GFZ for removing non-tidal atmospheric and oceanic variation.
• Level-2: JPL produces Level-2 data now at all times and not just for validation.
• Level-3: also includes the scaled gridded products produced by JPL. might want to just rename this to "spatial fields"
• "gridded" and " Anomaly (TWSA)"
• I think this line should be in the next section about formats
• should be "triangular"