Dates out of range or missing data
Created by: jdha
Need to throw a Warning if the src files do not cover the dates in the namelist - possibly with a summary output of the dates pynemo thinks are covered by the input files.
Also need to include the option of using the nearest neighbour to start/end a time series. e.g if we have 5 day mean input from day 3.5 in 1980, but no the last 5 day mean from 1979 - extrapolate for days 1 & 2.