Add slev obsdata download function

[veenstrajelmer]

Todo:

• download based on GTSM scripts with some updates
• solve code duplicates, e.g. create standardized subsetting function instead of one for each dataset, same goes for downloading. But in that case the source has to be derived from the geodataframe.
• x/y is generic because of geodataframe, also do for time_min/time_max/country
• re-enable kwargs for catalog and dataretrieval (like gesla dir or drop_noncoastal stations)
• add test cases
• including export of resulting obspoints, e.g. to xynfile
• drop non-geom lat/lon columns and non standard time columns
• names of nc files should correspond to xynfile, so add unique stationname to all gdf's and use that column in writing nc and xyn. Also remove name_column argument in xyn function. Also add unique stationname to expected columns test.
• possible to use gesla directly from zipfile or very bad for performance?
• Add station_name/ndays to be generic across datasets (and add to test_ssh_catalog_subset())
• check whether ioc contains more data than 1 day (first and last column seem to imply this)
• expose public functions with from observations import *
• check minimal required version for erddapy
• example notebook (current example scripts in gtsm repos) >> result
• update whatsnew.md

Follow-up: #712