Neuroglancer links for worst errors

[minnerbe]

I refactored the client that computes an error measure on every (same layer) pair of tiles in an aligned stack:

• it does not do comparison by default anymore, but a second stack to act as a baseline is optional;
• it does not write all the errors to disc anymore;
• instead, it logs the N worst pairs and assembles a neuroglancer link that shows exactly that pair.