create new sequence observing mode

[lspitler]

The aim is to run through a sequence of targets. This is useful for testing during commissioning or even for normal observing if, say we want to visit a calibration star, between two science observations of the same target.

Currently the POC scheduler always selects the highest-ranked (observable) target in its priority list, which might be the last target it just finished observing.

Instead in the sequence mode, we'd like to move onto another target even if the first one is still observable.

[lspitler]

Let's start with a sequence modification where you have an empty list at the end. And POCS I guess shuts down.

[jermainegug]

Making a new constraint Sequencer (please give any suggestions for a better name) in constraints.py which will make sure that current_observation is not one of the entries in observed_list. I am not too sure what we want the get_score function to return but for __str__ I have just have return "Don't repeat observations".

[jermainegug]

The sequencer has been merged into panoptes/POCS. @lspitler this issue can be closed.

I am not sure if @wtgee fixed the dwelling issue, but if not then we can make a new issue for that.

[jermainegug]

Make new sequencer constraint