Feature: add randomization to converting 360_day cal to standard

[dgergel]

Currently in the calendar module, when using the convert_calendar function to convert a 360_day calendar to standard (or vice versa), whether the date or year option is chosen for align_on, the same dates of the year are always chosen, with the only difference being between leap and nonleap years.

One potential issue with this is that it might affect seasonal cycle behavior and show up in annual cycle statistics. To deal with this potential problem, the LOCA downscaling project chose a random day within a 72 day block to fill in each year, so that different days were getting filled in such that it would have no discernible effect on the annual cycle stats. They also interpolated days rather than filling in a missing day.

It seems like this would be a really useful feature to add in to xclim's calendar module.