Changing Units (imperial/metric) should be disallowed or deferred while a file is in progress

[RobMackie]

I was cutting a file (on simulator) and testing the new display of units in the DRO.

I changed units mid-job. The job completed at 1/25.4th scale.

The early part of the opensbp file was translated using units of inches, and the latter part was generated using millimeters. (i.e. scaled down). Either the job should grab the current.

There are several possible solutions. I think the simplest and most understandable for our users at this point is to either ask the user to wait until the job is done or inform the user that the change will be applied after the current job completes.