Lsqfit

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Fixes GSL documentation. Improved makefile.

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- New setup.py to facilitate installations when numpy and cython
are not installed. Also cython is now optional. scipy is now
required (to guarantee that lsqfit will work even if the gsl
library is absent).

- Slight improvement to MultiFitter's plotting capabilities.

- Fixed interface to scipy.optimize.least_squares so ignores
if scipy version earlier than 0.17 (used to crash).

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Tiny changes:

- Another fix for a rare bug (an edge case) in MultiFitter.chained_lsqfit.

- Adds an extra feature in response to a request from a user
of the corrfitter module: lsqfit.MultiFitter has a new
keyword wavg_svdcut --- see the documentation.

- Minor documentation corrections.

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Very minor release:

- New option fitname in MultiFitter: fitname(x) creates new name
for chained fit link from old name x.

- Minor bug fix for MultiFitter.chained_lsqfit having to do with
svdcorrection.

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Fixes a bug that caused MultiFitter to crash in unusual circumstances.

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The only significant change is the addition of MultiFitter.

- Added lsqfit.MultiFitter which is a framework from organizing
fits around a small number user-defined of models that are fit
simultaneously to many different pieces of data using different sets of
parameters. It was developed to support the corrfitter module, but is more
general.

- Tiny bug fixes to wavg to cover certain edge cases.

- Faster tests.