Add ENSO analysis and plot scripts

[megandevlan]

Create an analysis script that computes a variety of ENSO metrics (western extent, duration, etc.) for a new case, adding that to a pre-created netCDF file containing the same variables across previous simulations. Those are then plotted and placed in a new "ENSO" folder on the ADF website.

[megandevlan]

@justin-richling I don't think there will be any issues, but wanted to check in before merging!