Add support for dithering

[sybenzvi]

This is a feature request moved from desimodel. Specsim should allow the user to dither the telescope given an observation spot in the focal plane. Some hooks are needed in the config and simulator modules. In principle I could implement them, but to avoid conflicts I'll wait for comments from @dkirkby.

[dkirkby]

There is already a configuration parameter we can use for dithering called instrument.offset.value but the logic in specsim.instrument.initialize() will need to be updated so that:

• either radial or (x,y) offsets can be specified, and
• a constant offset can be combined with interpolated offsets as a function of (r, lambda) and (x, y).

I don't think any other changes should be necessary (except for docs and unit tests).

[tolgayapici]

we added a parameter called fiber_placement in the fiberloss module. this change enables the dithering (the scripts are at desidithering) to work. by default, it is set to [0, 0] which makes all the normal operations uninterrupted. the changes are currently on dithering branch. can we merge with master?

[dkirkby]

Please make a pull request so I can review & test your changes.

[tolgayapici]

before I make a pull request, I realized that travis tests are failing for python2.7
I will try to find the problem first.

[tolgayapici]

The travis tests fail for transform.py (a file I have not touched) with astropy=development. The last test for the master was done 2 months ago. Is there any change that the astropy[development] has some issues? I have restarted the build of the master and the problem is there too (with transform.py). Do you want me to take a look at transform.py?

[dkirkby]

Thanks for looking into this. I will investigate eventually (and you are welcome to take a look also!)