aggregating ungridded data should produce ungridded data

When aggregating ungridded data, coordinates that are not referenced in the command should be averaged over the aggregation box and produce ungridded (though aggregated) data.

E.g.

cis aggregate AOT_440:<aeronet file> t=[2007-01-01T00,2008-01-01T00,PT6H]
should aggregate the data over time. lon & lat should be averaged over this time. (Since this is station data, lon & lat will not change).

In future, we will also have Maritime Aerosol Network data. These are data from (moving) ships but very similar to AERONET. Averaging lon & lat over the time aggregation domain will produce a time-averaged position for the ship.

cis aggregate nfine:<gassp file> t=[2007-01-01T00,2008-01-01T00,PT1H]
should similarly aggregate the variable nfine and the coordinates lon/lat and alt over 1 hour