Add allDatasets query method

[rsignell-usgs]

In my ERDDAP time series explorer, I used the existing erddapy method for advanced search , but also needed to use an allDatasets query to return the lon/lat values of all stations. I did this with an ugly erddap URL string formatting:
https://github.com/reproducible-notebooks/ERDDAP_timeseries_explorer/blob/master/ERDDAP_timeseries_explorer.py#L197-L200

Seems like the allDatasets query might warrant a method, since it's a bit different than a regular datasets query

[rsignell-usgs]

I just realized we don't need a special allDatasets query method, because allDatasets is just a special dataset, so we can use something like:

    kwargs = {
        'minTime<=': stop_time,
        'maxTime>=': start_time,
    }

    variables = ['datasetID', 'minLongitude', 'minLatitude']
    url = e.get_download_url(dataset_id='allDatasets', variables=variables, response='csv', **kwargs)

to generate ERDDAP URLs like this one, which returns the ID, min_lon and min_lat for all datasets in a certain time range

https://erddap-uncabled.oceanobservatories.org/uncabled/erddap/tabledap/allDatasets.csv?datasetID,minLongitude,minLatitude&minTime%3C=2018-01-23T12:52:10Z&maxTime%3E=2018-01-16T12:52:10Z

[ocefpaf]

url = e.get_download_url(dataset_id='allDatasets', variables=variables, response='csv', **kwargs)

We should document that.

[ocefpaf]

I forgot to document this. Re-opening so we can do that.

[ocefpaf]

The docs were added in vinisalazar@f0ec8a6. Closing this now.