Add example for creating pixel mask to ophys tutorial (inspired by #572)

[ehennestad]

Motivation

It was unclear how to create pixel_mask for PlaneSegmentation objects. This was raised in issue #572

How to test the behavior?

open ophys.mlx

Checklist

• [x] Have you ensured the PR description clearly describes the problem and solutions?
• [x] Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• [ ] If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[ehennestad]

@bendichter @lawrence-mbf one test fails, related to nwb version. Is that due to matnwb still being on v2.6.0?

[lawrence-mbf]

@bendichter @lawrence-mbf one test fails, related to nwb version. Is that due to matnwb still being on v2.6.0?

Yeah, the message implies that pynwb is using 2.7.0 by default.

[bendichter]

@ehennestad this looks great! Just one minor edit, then it's good to go

[ehennestad]

@bendichter