Update nbmake markers in notebook metadata

[mcflugen]

Description

This pull request updates the nbmake markers we use for the notebook tests that label certain notebooks as "slow". The markers are now in their own namespace. The markers used to be in the notebooks' metadata as,

"metadata": {
  "execution": {
    "markers": "slow"
  }
}

but are now,

"metadata": {
  "execution": {
    "nbmake": {
      "markers": "slow"
    }
  }
}

Checklist - did you ...

• Add a news fragment file entry if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?
• All tests have passed?
• Formatted code with black?
• Removed lint reported by flake8?
• Sucessful documentation built? (if documentation added or modified)

[coveralls]

coverage: 72.569% (-0.002%) from 72.571%
when pulling 9e0839e on mcflugen/update-nbmake-markers
into de0d762 on master.