Implement ForecastIndex (an xarray custom index) #87
Description
As flagged in #47, #38 and #33, the long awaited flexible/custom index functionality is now available in xarray (see the v2025.07.1 release notes). See the SciPy presentation and custom index gallery for details.
There is already a custom index for handling forecast data - the rolodex.ForecastIndex described at this gallery page.
The gallery page talks about the fact that there are four different ways of constructing views of forecast data (or more specifically, of “Forecast Model Run Collections; FMRCs"). By using the rolodex.ForecastIndex we can efficiently select and slice data along any of those four views, essentially doing away the cumbersome functions we've written in our array_handling.py module for swtiching views of the data.