Automatic detection of minimum g vector magnitude is slow and unreliable

[JJPPeters]

When the program determines the smalled g-vector (to then determine an appropriate mask size) this algorithm used is slow (particularly for large images) and unreliable.

A temporary balm for this issue is to have the dialog giving the user the option of typing in a mask size or clicking on a vector show first. An option can then be added to attempt to determine the g-vector is the user wishes whilst other users can quickly type in the vector without having to wait for the automatic detection to complete.