NCEP TDS links outdated/inconsistent

[dcamron]

In reviewing some support tickets for separate issues, I discovered that multiple notebooks across the gallery and workshop

pages/gallery/500hPa_Vorticity_Advection.ipynb
pages/gallery/850hPa_Temperature_Advection.ipynb
pages/gallery/Ageostrophic_Wind_Example.ipynb
pages/gallery/declarative_500_hPa.ipynb
pages/gallery/HILO_Symbol_Plot.ipynb
pages/gallery/miller_composite.ipynb
pages/gallery/MSLP_temp_winds.ipynb
pages/gallery/Smoothing_Contours.ipynb
pages/gallery/xarray_500hPa_map.ipynb
pages/workshop/MetPy_Advanced/QG Analysis.ipynb
pages/workshop/MetPy_Case_Study/MetPy Case Study.ipynb

attempt to access data or guide users with outdated ncei.noaa.gov/thredds/<access>/grid/<product> data access URLs for GFS/NAM/NARR products. These can now generally be found at ncdc.noaa.gov/thredds/<access>/<product> (NCDC data access) which was correctly described in some of these same notebooks. If this was already known or expected, let me know; I didn't see anything on this elsewhere!