Gvar

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Very minor fixes.

- Small improvement to gvar.chi2(...).

- Default svdcut changed to 1e-12 everywhere.

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Very minor enhancement.

- gvar.linalg.lstsq now supports both weighted and unweighted least squares
for solving a x = b when b has a covariance matrix.

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- New functions eigh, svd, lstsq added to gvar.linalg. Also have a faster
version of solve.

- gvar.load and gvar.dump now work with yaml as well as with pickle and json.

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Bug fixes.

- Bug fix in evalcov_blocks() that affects some Windows installations
(mismatched integer types in cython code).

- Bug fix for ode to deal with 0(0)/0(0) fringe case.

- Minor documentation fixes.

- Bug fix in GVar.__richcmp__ for Python 2.7 on macOS 10.13.5.

- Default svdcut=1e-15 added to raniter. Having a small non-zero
svdcut is standard elsewhere in code.

- Improved handling of cython in makefile.

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Minor update to keep up with changes in numpy.

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- More improvements for setup.py.