Add option to fill missing leap-days

[gutmann]

This is a feature request to add an option for metsim to fill missing days by interpolation of the input data in time, including changing the calendar from noleap to standard.

Some forcing datasets do not have leap days (many climate models and even daymet...?!?). While metsim supports a noleap calendar (thankyou!), many models that use its output do not.

A nice feature would be an option for metsim to read data on a noleap calendar, fill missing leap days by interpolating the Tmin/Tmax/Precip/wind data between Feb 28 and Mar 1, then apply MTCLIM algorithms, and output data on a standard calendar.

This might implemented using xarray's interp option, though I think that adds a dependency on scipy. Leap days at the very beginning or end of the requested time period can be ffill and bfilled