Vegas

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Starting with this version, vegas requires module gvar ('pip install gvar') in
order to function. In the past it used a (very poor) substitute for gvar if
that module was not installed. That kludge is not sustainable.

- Integrand functions in the past could return numbers or numpy arrays. Now
they can also return dictionaries whose values are either numbers
or numpy arrays. This is useful in situations where multiple integrals are
being done simultaneously since the keys can be used to document and keep
straight which integral is which. This should make code easier to read and
write.

- Miscellaneous internal changes to simplify the vegas code that handles
integrands.

- Switch to pip for installing and uninstalling. It is more robust than
the alternatives.

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Very minor fix: changes in numpy v1.9 broke MPIintegrand. It is now
fixed.

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This is a minor bug fix release.

- Fixed a bug that caused option adapt_to_errors to malfunction. The
malfunction caused the integral to be less than optimal, although
still correct.

- Cleaner implementation of the (Feynman) path integral example.

=========================
There is a new decorator, vegas.MPIintegrand, that makes it trivial to
evaluate integrands on mupliple processors using MPI. See the tutorial section
on Multiple Processors for more discussion. This is only useful for
integrands that are expensive to evaluate.

Beginning with this version of vegas, version numbers will be consistent
with the rules of "semantic versioning" (http://semver.org). The most
important consequence is that any change that is *not* backwards compatible
with the current interface (as defined by the documentation) will be signaled
by a change in the major version number, where the full version number has the
format major.minor.patch (so the major version number is 2 for version 2.1.4).
A side effect of this policy is that increases in the major version number can be
triggered by relatively obscure changes in the interface that have no
effect whatsoever on almost all existing codes. The minor component increased
for this version because it adds new functionality; but it remains compatible
with the previous version.

============================
More small changes (all backwards compatible).

- vegas.Integrator has new parameter ran_array_generator. This is the random
number generator used by vegas. The default function is numpy.random.random.
The new parameter allows users to substitute a different random number
generator, if they wish.

- Legacy name vegas.VecIntegrand now works. This isn't needed for new code
(use vegas.BatchIntegrand), but saves having to update old code.

- The path-integral example is a little bit more organized.

- Ongoing polishing of documentation.

=============================
The last (probably) in a series of very minor changes. Here only the
documentation has been changed. There is a new section in the Tutorial
showing how to use multiple CPUs to evaluate costly integrands in parallel.
There is also a new section on how to use vegas to do multi-dimensional sums,
rather than integrals.

Also fixed errors in the documentation for random and random_batch generators.