CAS - DAP authentication where possible

[JimCircadian]

Was going to implement this for the ORAS5Downloader but ran into problems with xarray using verify on pydap when it's not yet available.

Add this option in the future, using an alternative for the moment:

>>> from pydap.client import open_url
>>> import xarray as xr
>>> cas = "https://cmems-cas.cls.fr/cas/login"
>>> session = setup_session(cas, <<REDACTED>>)
>>> dataset = "global-reanalysis-phy-001-031-grepv2-daily"
>>> dap = "https://my.cmems-du.eu/thredds/dodsC/{dataset}".format(dataset=dataset)
>>> store = xr.backends.PydapDataStore.open(dap, session=session)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jambyr/anaconda3/envs/icenet/lib/python3.8/site-packages/xarray/backends/pydap_.py", line 114, in open
    ds = pydap.client.open_url(
TypeError: open_url() got an unexpected keyword argument 'verify'
>>>