--inter-store keep not working

[kai-polsterer]

when using the "--inter-store keep" option there are no additional SOM files produced. This might be related to FIX #17: and ISSUE #22.

[BerndDoser]

Fixed by 343eeb8

It was a bug in the ProgressBar. The valid function was not working,
because next_progress_print was updated within the increment operator.
This works for printing the progress bar but not for the later valid()
function, which is used for writing intermediate results.

Fixed by change from comparison with next number to modulus.

Added some unit tests and output stream.