CAS-14283: Fix bug with initial residual and deconvolve
When the initial residual was made, `deconvolve` was called with `self._niter`,
and performed actual iterations before the GUI popped up. This resulted in the
initial model image being populated, as well as other side-effects such as
cyclethreshold etc. being computed.
The net result was that the first user-defined trigger for the major cycle
would more often than not run zero or one iterations and quit, since the peak
is already close to the cyclethreshold.
