Add tool to fix broken Paintera project

[oflynne]

It just happened that some mipmap levels in a label source contained broken blocks triggering EOF exceptions that then triggered Runtime exceptions that then triggered looming objects on the heap (precise details unknown). Removing the mipmap levels fixes the exceptions but also means that the mipmaps are inconsistent and have to be regenerated. This can be 'fixed' by painting into higher mipmap levels and committing but it takes a long time and is weird. Better would be a tool that we can send over an existing Paintera project that checks for broken things and fixes them (in this case, regenerating the mipmap level from the previous level would have worked but no dedicated tool to do exactly that exists in either n5-spark or label-utilities-spark, and they are also more difficult to use).