add support for dtype of station data

[miniufo]

GrADS support a type of station data. Not so sure if someone need to read in the station data. Since there is no objective analysis method like oacres, plotting the (randomly distributed) station data is not straightforward. Besides, this is a less frequently used case than regular gridded data.

Do we need to add support to read in this type of data?

[hechentao]

Reading station data is not supported now? I think this function is very important!

[miniufo]

Not yet. Just feel that the station data format is uncommon in recent years. It is actually a list of (lon, lat, data), so it can be represented quite well by pandas.DataFrame.