Improve long names for AOD fields

[acollow]

Users are reporting confusion pertaining to AOD variables in MERRA-2 and asking for clarification between extinction and AOD. A minimally disruptive solution would be to update the long names. While updating variable names to, for example duaod550 instead of duexttau, would be beneficial, it would disrupt analysis as it makes it difficult to compare new and old versions.