Warn if a cleaving task was assigned to another user

[hubbardp]

Lowell will add to the task JSON a field with the login name of the user to whom the task was assigned. Philip will update the Neu3 taskbodycleave.cpp code to check that field, if present, against the name of the user running the Neu3 session, and produce a warning dialog if they don't match.

[hubbardp]

Now in the "neu3-body-cleave-error-handling" branch:
https://github.com/janelia-flyem/NeuTu/tree/neu3-body-cleave-error-handling

[hubbardp]

The "neu3-body-cleave-error-handling" branch is waiting to be merged into the "develop" branch:
#71

[hubbardp]

Merged.

[hubbardp]

Reopening, because the warning appears and has to be dismissed multiple times for a multi-body assignment, and that is annoying.