Fix NUTS Slice Criterion

Summary
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Bad parentheses and inadvertent casting.  NUTS was implementing the slice criterion as

util.log_u + (h - util.H0) < 0

which computes a check on (h - until.H0) and then adds until.log_u before casting to a bool.
This pull request fixes this to

(util.log_u + (h - util.H0) ) < 0

so that the slice is taken into account correctly.

Intended Effect
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Proper check of the slice criterion.

How to Verify
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The slicing behavior (both for an accepted slice and the max_delta condition) are checked in the accompanying unit test.  Run with mkfast test/unit/mcmc/hmc/base_nuts.

Side Effects
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No user-facing effects, will cause small differences in sampling behavior for some models.

Documentation
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None.

Suggested Reviewers
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Anyone, but @syclik should confirm if the unit test is sufficient.