Improve readability by packing frequently used variables into namedtuples.

[zoj613]

closes #67

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[zoj613]

@rlouf could you please help diagnose the one failed test test_nuts_mcse? It appears that the MCSE fails for one of the cases in the NUTS sampler. The changes seem fine and do not appear to change behavior so i'm a little confused.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (75fd205) 100.00% compared to head (53f8f8b) 100.00%.

Additional details and impacted files

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[brandonwillard]

These updates look great; I'm ready to move forward with them.

[zoj613]

These updates look great; I'm ready to move forward with them.

I will still need to look into updating the docstrings. @rlouf any thoughts on these changes?

[brandonwillard]

Merged, because these changes are already very helpful. We can add follow-ups for the docstrings and anything else.