Fix bugs with Piecewise process output shape

[drasmuss]

Motivation and context:
This fixes two related bugs with the Piecewise process:

1. Piecewise({1: [1, 1, 1]}) would return 0.0 for times < 1 (rather than [0, 0, 0]).
2. Piecewise({1: 1}) would return an array with shape () for times < 1 and shape (1,) for times >= 1,
   because ravel upsizes its inputs to at least 1d. This doesn't really produce any errors, but it's
   probably good to be consistent. I thought about adding some logic to check if the output shape
   should be () or (1,), but then figured that it was probably easier/more consistent to just always have
   the output of the process be an array, instead of a scalar.

I wasn't sure whether to add a changelog entry or not. Technically a bugfix, but it comes so closely on the heels of the feature release that it seems unnecessary (anyone looking at the changelog would probably be seeing both things for the first time).

How has this been tested?

Added a new test, see test_shape_out

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Where should a reviewer start?

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• [n/a] I have updated the documentation accordingly.
• [n/a] I have included a changelog entry.
• I have added tests to cover my changes.
• All new and existing tests passed.

[tbekolay]

LGTM! Agreed that it shouldn't be a new changelog entry; I pushed a commit that lists this PR in the existing changelog entry, though, as I'll do for #1355. If my new commits look good to @drasmuss I'll merge!

[drasmuss]

Looks good, and that makes sense with the changelog entry.