Add time uncertainty

Currently, the tool assumes that the time is known exactly from perfectly accurate image metadata. This may be a reasonable assumption for imagery taken on internet-connected mobile phones with the metadata preserved, but there may be situations where only an approximate time is known.

It would be good if this uncertainty could be represented, perhaps with a broader band.

An outstanding question is how to do this in a computationally efficient way? We should avoid recomputing shadow lengths across the entire globe for different moments in time, if it can be avoided...

No files, tests, or entry points are named. Start by locating the command-line code that computes shadow lengths from image time metadata, then determine how approximate times should be represented and how to avoid recomputing the globe for every moment; done means uncertainty is supported efficiently.

Written by the indexing model from the issue text.