Add scikit-rf's time gating demo (#minor)

[xhluca]

Issue for app: #[issue number here]

App pull request

• This is a new app
• I am improving an existing app (redesigns/code "makeovers")

About

• Playground deployment URL (new version):
• Current gallery app URL: (delete this line if inapplicable)
• Python app repository link: (delete this line if you are working on a Python app)

Workflow

• I have created a branch in the appropriate monorepo, and the
  elements necessary for successful deployment are in place.
• If the app is a redesigned and/or restyled version of an
  existing gallery app, I've summarized the changes requested in the
  appropriate Streambed issue and confirm that they have been applied.
• If the app is on the Dash Gallery portal, I have added a link to
  the GitHub repository for the source code in the portal description.
• If the app is a reimplementation of a Python gallery app for the
  DashR gallery, the app in this PR mimics, as closely as possible,
  the style and functionality of the existing app.=
• I have removed all Google Analytics code from the app's
  assets/ folder.

The pre-review review

I have addressed all of the following questions:

• Does everything in my code serve some purpose? (I have removed
  any dead and/or irrelevant code.)
• Does everything in my code have a clear purpose? (My code is
  readable and, where it isn't, it has been commented appropriately.)]
• Am I reinventing the wheel? (I have used appropriate packages to
  lessen the volume of code that needs to be maintained.)

Post PR (at merge time)

• When you are merging, make sure to write one of the following tags in the commit message (or it will default to patch):
  • #patch - An app has been updated or fixed
  • #minor - A new app has been added, or an app has been significantly reworked
  • #major - Breaking changes, make sure to discuss with dash-core before using this tag