Specsim

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- No changes yet.

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- Update values of support width, read noise and dark current to match the desi.yaml file in $DESIMODEL (PR 121).
- Fix unit test errors on astropy >= 5.1 (PR 124).

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- Move fastfiberacceptance into desimodel (PR 114).
- Fix non-catastrophic exception flagged in `issue 119`_ (`PR 120`_).

.. _`issue 119`: https://github.com/desihub/specsim/issues/119
.. _`120`: https://github.com/desihub/specsim/pull/120

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- support astropy4 in unit tests, drop py2.7 and 3.5 testing (PR 111).

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- Changes to SimulationExamples notebook:
- Change median LRG minor/major axis ratio from 0.16 to 0.5.
- Add quantitative comparison of galsim and fastsim fiberloss modes.
- Streamline array indexing for downsampling and convolution operations.
- Do not require fastsim file to be present (fixes 90).
- Update to astropy_helpers v2.0.6 (merges 85).
- Remove python 3.4 from travis test matrix and update "old" numpy versions used (fixes 93).

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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).