Allow NetcdfDataset to open remote NcML Files

[lesserwhirls]

fixes #800

[cofinoa]

@lesserwhirls This PR diverges from how the NCML service type detection is made for the file protocol:

https://github.com/Unidata/thredds/blob/v4.6.8/cdm/src/main/java/ucar/nc2/dataset/NetcdfDataset.java#L697-L708

may be it woul better to do something like this:

    if (svctype == null) {
      //There are several possibilities at this point; all of which
      // require further info to disambiguate
      //  - we have file://<path> or file:<path>; we need to see if
      //    the extension can help, otherwise, start defaulting.
      //  - we have a simple url: e.g. http://... ; contact the server
      if (leadprotocol.equals("file")) {
        svctype = decodePathExtension(trueurl); // look at the path extension
      } else {
          svctype = disambiguateHttp(trueurl);
          if ((svctype == null || svctype == ServiceType.HTTPServer) && decodePathExtension(trueurl)==ServiceType.NCML)
              svctype = ServiceType.NCML;
      }
    }

in case the svctype is null or is HTTPServer, try with decodePathExtension

The other option it's would be to implement your appoach by reading the head and discover the ncml namespace ... or make both just in case.

[lesserwhirls]

Thanks for the feedback @cofinoa!

The idea behind reading the first bit of the file is like the IOSP "fail fast" approach, especially since we would possibly be trying to download something. However, maybe it's not a big deal given we are looking at an NcML file (which are typically small). Things started to get a bit crowded, so I offloaded the code to checkIfRemoteNcml.