Add a metric for coverage staleness

[mankoff]

The coverage metric used in the paper deals with missing data. It could be useful to better understand which data is missing and for how long. For example,

• Are there some pixels that appear in only one velocity product, and then are filled in at all other times?
• If there are four sequential time stamps (see Table), is it the same 50 % missing at both times? Or is the 50 % missing (covered) at time t1 swapped at time t2? In the former case, the discharge estimate is less robust. The 50 % coverage filled in at t1 and t2 is interpolated from t0 to t3. In the latter case the discharge estimate is more robust. The 50 % coverage filled in at t1 is only interpolated from t0 to t2, and the 50 % coverage filled in at t2 is only interpolated from t1 to t3.

Time	Coverage
t0	100 %
t1	50 %
t2	50 %
t3	100 %

A metric to track the duration of coverage gaps for each pixel (staleness) could help improve our uncertainty estimate.