Lsqfit

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A minor bug fix and a new diagnostic tool.

- nonlinear_fit.residuals is now an array of floats (not GVars), as it should be.

- Adds method nonlinear_fit.plot_residuals for plotting (normalized) residuals.

- Adds residuals to MultiFitter chained fits.

- Adds new method for making Q-Q plots of fit residuals: nonlinear_fit.qqplot_residuals().

- New documentation about inspecting fit residuals.

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This is another very minor update: a few tweaks, a bug fix, and a small feature update to
Multifitter.

- Minor tweak to how the stopping condition is reported.

- Minor tweak to *** labeling in nonlinear_fit.format().

- Minor bug fix in Multifitter (fixes wavg_kargs). Also now allows dictionary
entries in lists describing parallel fits.

- Documentation tweaks.

- Updates mpi examples for changes in mpirun.

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Minor update.

- New keyword arguments in lsqfit.nonlinear_fit: add_svdnoise, add_priornoise.
These are useful for evaluating fit quality. See new sections in
the documentation (Tutorial) on Goodness of Fit and SVD cuts (with
insufficient statistics).

- Keyword add_priornoise replaces keyword bootstrap in
simulated_data_iter and simulated_fit_iter. Old keyword still works,
for legacy code.

- Improved (faster) code for nonlinear_fit.simulated_fit_iter.

- Cleanup extraneous code.

- Now compatible with gvar v9.1.

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Improvements to lsqfit.MultiFitter.

- New wavg that can average arrays of different shape (but same dimension).

- New option (wavg_all) for chained fits to average results from all the
fits at the end, to restore (some) correlations, when fast=True.

- New fit.format() for chained fits that includes summary information
about individual fits in the chain.

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The extend parameter removed from nonlinear_fit; the associated
functionality (non-Gaussian priors) is now available automatically. The new
implementation, which resides entirely in gvar.BufferDict, is much simpler
and more robust. This causes changes in a variety of pieces of code, but will
have little or no effect on code that didn't explicitly use extend (and that
code should just stop referencing the keyword).

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Another minor bug fixed (in wavg) that can cause problems in MultiFitter.