Version 3 Soft Release

[smattis]

This PR contains all of the updates for the soft release of version 3.0. This PR, mostly maintains backward compatibility with version 2.0, but it is not guaranteed, especially in cases of saving and loading.

Major attributes are:

• Initial and updated densities are added to bet.sample.sample_set_base objects. Densities can be defined by Voronoi tessellations, random variables with certain statistics, Gaussian mixture models, Gaussians, or weighted Kernel Density Estimates.
• Automatic evaluation of PDFs and marginal PDFs can be done.
• The addition of support for density-based data-consistent inversion. Closes data-consistent methods #365. Most of this is in bet.calculateP.calculateR.
• Examples are restructured into "measure-based" (the old method), "density-based", and "bet_features". New examples are added. Runnable scripts in examples now begin with BET_.
• Notebook examples removed from repo. They are being moved to a new repo of notebooks.
• bet.sampling.adaptiveSampling is completely removed. Closes Deprecate adaptive sampling? #366.
• bet.sampling.basicSampling is completely overhauled to handle any type of random variable of type scipy.stats.rv_continuous. Closes sampler class options #362.
• Plotting methods for density-based methods.
• Changes how saving and loading of objects is done. Now uses pickle and save_object and load_object functions in bet.util. Closes why no pickle? #360.
• Testing is done with pytest. Closes nose deprecation #364.
• bet.postProcess.compareP is overhauled with new functionality and support for many types of sets. Comparison of marginals is added.
• Installation is now done with pip. Closes How we do installs #356. Versions for requirements are enforced and worked. Closes TODO: un-lock scipy #357.
• Interface with LUQ is added with bet.sampling.useLUQ. LUQ-type cluster data structures are supported for density-based inversion.
• Tests updated and simplified.
• Automatic testing now done with only Python 3.6 and 3.7
• Docs and Readme updated, cleaned, and simplified.
• compute_QoI_and_create_discretization deprecated. Going forward use compute_qoi_and_create_discretization.
• Closes 5/19 meeting #379

[eecsu]

As a general comment in relation to something Michael brought up, I agree that the plotting file-extensions should all be .png for the examples. Better for anyone using this in windows.

[smattis]

As a general comment in relation to something Michael brought up, I agree that the plotting file-extensions should all be .png for the examples. Better for anyone using this in windows.

Handled in #393

[smattis]

@eecsu Should be ready for you to look at

[smattis]

@eecsu See af59f09...03e52c0 for a diff between the current version of the branch and the last commit before your review.