Periods not in use are not printed to steps_in_timeline.csv

[jkiviluo]

I was trying to run a model using only one period out of the three periods defined. However, flextoolrunner.py does not output those 'unused' periods to stes_in_timeline.csv' and this results in 'not within specified set' error when some data has been defined also for those unused periods.

The 'runner.write_full_timelines' call (line 1959) should pick all periods and not just the ones in use.

[jkiviluo]

Issue #235 has a solution for the problem that we don't know what all the periods can be. Model class should have a new parameter periods_available (see the issue for more).