Add additional data sources

[mcflugen]

This pull request adds the ability for a user to specify a data source from which to fetch WaveWatch III data. Users can choose to get data from the following sources,

• https://www.ncei.noaa.gov/thredds-ocean/catalog/ncep/nww3/catalog.html
• https://polar.ncep.noaa.gov/waves/hindcasts/multi_1/
• https://polar.ncep.noaa.gov/waves/hindcasts/nww3/