Specsim

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- Update centroid offset config to match desimodel 0.9.6. (PR 91).
- Manage random state for centroid offsets (fixes 83).
- Update jupyter notebooks (supercedes 89, fixes 76).
- Update API docs (fixes 39).
- Implement optional Gaussian noise realization to decouple noise from mean value (92).

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- Changes to test infrastructure to support Astropy 2, no API changes.

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- Implement fast fiberloss calculator that interpolates GalSim results (78).
- Add uncorrelated random offsets (81)

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- Update to astropy_helpers v2.0.1 (79).
- Handle non-backwards compatible changes to astropy.constants in astropy 2.0.
- Handle deprecated longitude, latitude attributes for astropy >= 2.0.
- Remove tests against python 3.3 and numpy 1.7.

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- Fix Simulator output table units with astropy 1.3.2 (62).
- Add support for simulating calibration exposures (54).
- Add wavelength min/max options to Simulator.plot().
- Better support for alternate simulation wavelength grids (PR 60).
- Add Simulator option to not create and fill per-camera output tables (PR 67).

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This version adds the ability to specify per-fiber positions and source
properties. Fiber acceptance fractions can now be calculated on the fly
(using GalSim) to simulate realistic throughput variations across the
focal plane.

- Add support and travis testing for python 3.6.
- Add fiberloss module with support for more realistic fiberloss calculations.
- Add optional dependency on galsim (only required for on-the-fly fiberloss
calculations).
- Add jupyter notebooks to document the Simulator and fiberloss calculations.
- Update the simulate() method to take optional array parameters specifying
per-fiber positions and source properties.
- Allow the observation time and pointing to be changed after initialization.