Document encoding of percentile-over-time measures

[pp-mo]

It seems that when PDT4.10 is loaded, it generates a special scalar aux-coord called "percentile_over_time" containing a percentile value.

PDT4.10 is described as "Product definition template 4.10 – percentile forecasts at a horizontal level
or in a horizontal layer in a continuous or non–continuous time interval"

Loading is done here and here

Saving is also supported here, here and here

But we don't support unrolling a percentile dimension on save, (you can split it up for yourself)

None of this is documented,
we should at least do that.