Vegas

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- More robust treatment of round-off errors when calculating means and variances in vegas.

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- Adds PDFIntegrator.sample() which creates (weighted) samples
from the integrator's probability distribution (eg, for making
histograms or contour plots of probability distributions).

- Fixes defect in handling of round-off errors (in sigf).

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Move vegas.pxd to its proper place.

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- Default value for vegas.Integrator parameter mpi is now False (was True) to avoid
irritating security prompts on MacOS. Thus mpi=True must be specified if the plan
is to use mpi. The default can be changed back to the old default using:
vegas.Integrator.defaults['mpi'] = True.

- Fixes bug in Integrator.synchronize_random that caused crashes when using MPI.

- As of Version 5.6, vegas.Integrators can be configured so that integrands are called
with dictionaries rather than 1-d arrays -- so f(xdict) rather than f(x1darray).
Now the arrays can have any shape (ie, not just 1-d).

- Fixes bug that caused vegas to crash for integrands returning arrays with more than 1 dimension.

- vegas.PDFIntegrator has a new method stats for calculating the means and standard
deviations of arbitrary (multi-dimensional) functions f(p) with respect to a PDF.

- vegas.PDFIntegrator(pdfitg) where pdfitg is a PDFIntegrator now works.

configured to package integration points as dictionaries rather than arrays.
This is to facilitate integration over (large) heterogeneous
collections of integration variables. See the tutorial section
on Dictionaries.

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- New default random number generator, to deal with
changes in numpy version >=1.17. Now tied explicitly to gvar's
random number generator (which is from numpy).

- Simpler and more robust implementation for vegas.__version__
(doesn't use importlib.metadata, which is buggy).