DAP4 support?

[FObersteiner]

Since I just disabled the dap2 service for the THREDDS TDS I maintain, I noted that siphon does not allow the newer dap4 for remote access:

        if service not in (CaseInsensitiveStr('CdmRemote'), CaseInsensitiveStr('OPENDAP'),
                           CaseInsensitiveStr('DODS')):
            raise ValueError(service + ' is not a valid service for remote_access')

[ref] - is there any specific reason for this?

Since xarray does support dap4 via pydap, it would be nice if it would be available via siphon as well. From a quick look at the source code, this does not seem to be overly complicated although I'm not a siphon power user so please excuse my ignorance ;-)

One small caveat I noted is that TDS seems to pass dap4 URLs with the http(s) prefix instead of dap4. This needs to be corrected before passing the URL to xarray.