Timing constraints need reworking

[pahjbo]

Timing constraints should consider when and duration separately - In addition it would be good to make it easy to allow for periodic observations with varying cadences.

The GUI should be adapted to make this easier.

• The meaning of the AVOID button was not clear in the GUI or model really
• would be useful to perhaps be able to specify times in LST/HA
• avoid twilight - anyway this sort of thing is important for optical observations.