Fix December 1 leap year interpolation error

[griff-rees]

No description provided.

[RuthBowyer]

From checking out the max vals per day of tasmax, and plotting the hashed lines as follows, I think we're OK and it's just the first value in the series that is much higher. So just something to note for future users of this method.

Grey line = day 365, pink line = day 1095, green line = day 1460 -- lines around the first day of each new year but with a day or 2 day lag

Will check tasmin and precip