ENH: Update reported sampler diagnostics for pm.Slice() step method

[AlexanderFengler]

Before

with model:
    pm.sample(step=pm.Slice())

After

with model:
    pm.sample(step=pm.Slice(full_diagnostics = True)

Context for the issue:

Currently the pm.Slice() step method doesn't seem to include the final slice-widths in it's diagnostics. Including either the final w array after tuning, or draw-wise could help understand the sampler behavior better and help users with settings after some initial test-runs.

If ram is an issue, it may be an option to set an extra argument as suggested above.

[welcome]

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[ricardoV94]

Sounds fine to me