Fix S3 NcML datasets

[Tara Drwenski]

To reproduce this issue:

• Using TDS test catalog, go to dataset http://localhost:8081/thredds/catalog/s3-dataset-scan/ncml/nc/namExtract/catalog.html?dataset=testS3DatasetScan/ncml/nc/namExtract/test_agg.ncml
• Try to use OPeNDAP or NCSS, both will return an error "java.io.IOException: Cant read cdms3:thredds-test-data?ncml/nc/namExtract/test_agg.ncml#delimiter=/: not a valid CDM file."

This error occurs because the convoluted logic for parsing Url strings in DatasetUrl::findDatasetUrl cannot recognize a S3 NcML file as NcML.