normalize the display of leap years in doy plotting methods

[12rambau]

In the current implementation we simply drop the dayOfYear value which means the days are not corresponding after the first of march when we display leap and non-leap years. This should be normalized during the computation of non-leap year doy.

My main concern is that if we do so, the "earhengine" way of computing date (with doy + year) will be broken. So I think it should only be changed in the plotting methods and kept as it is in the GEE computation. It also need to be explained in the documentation.