Download multiple days from CDS api

[leifdenby]

@sjboeing suggested this might speed up downloading. We might also hit a ceiling on max number of requests if we don't do this I think, no?

[sjboeing]

I think we might indeed run into issues if we try to download the domain for a whole month: so far I have downloaded up to 4 days (a typical trajectory) without trouble. The drawback of downloading multiple days and then splitting is that in some cases the data will become too big for online conversion to netcdf (though downloading grib and converting offline would still be possible), so if this is done it should probably be optional.
Another intermediate solution is to limit the number of requests that are submitted, and then wait for these to download before proceeding.