Forbid slashes in names of Groups and Datasets

[bendichter]

• add checks to ensure that name and default name args do not contain slashes.
• include tests

Motivation

We want to make sure people don't create extensions that contain names or default names that contain slashes.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.00%. Comparing base (a3c1998) to head (3fef9ec).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1219   +/-   ##
=======================================
  Coverage   89.00%   89.00%           
=======================================
  Files          45       45           
  Lines        9965     9969    +4     
  Branches     2831     2833    +2     
=======================================
+ Hits         8869     8873    +4     
  Misses        774      774           
  Partials      322      322           

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