existing data gets overwritten if filename stays the same

[bvreede]

There is currently no check that confirms if a filename already exists or not. That means locally existing data may be overwritten if the same filename is generated.

In general this may be an unpleasant surprise for some users. Also, because months and pressure levels are not part of the filename, a new call with changes in these parameters will download a datafile that will overwrite existing data. In this case, recommending to a user that they could change the prefix could be an option.

While this issue may not have priority to be addressed (or perhaps we choose not to address it at all), it could be done in tandem with #68.