Document the interpolation of gridded data to NUTS regions

[charliepascoe]

Document how the interpolation from gridded data to regional averages (for NUTS regions) is done,
especially where an adjustment is made to prevent small regions (e.g. Hamburg) from appearing to be blank.

The information about the interpolation methodology will appear on a stand-alone page in the "help" section of the ECDE website. Therefore the explanation can be as long as is needed and diagrams can be used to illustrate the interpolation method. This information page can then be referenced from the explore page of the climate indices.