MeshConverter: Check Units before Run

The MeshConverter did not check the unit to be valid before starting the interpolation. This means if the unit was invalid, this would only appear after the full interpolation process has finished. This MR fixes this.

Furthermore, when using a log file, any exception thrown lead to a hard abort of the program, because the log file went out of scope with the exception, but the logger still tried to access it. Also fixed.

Finally, I added a short sentence about how to deal with mandatory units fields when one really wants to have sth like "unit potential".