Stop code if factor for filter preconditioner out of range

Description

If the factor for the filter preconditioner is below 0.5 or above 1.0, stop the code because it might produce wrong results.

I haven't added a test because this code is only executed when the code fails. Thus expect the code coverage to go down.

Fixes #158 (closed).

Checklist

• I have checked that my code follows the Octopus coding standards