Adding rob_blobbogram()

[rdboyes]

PR Checklist

All Submissions:

• [ x ] Have you followed the guidelines in our Contributing document?
• [ x ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ x ] Does your submission pass R CMD check locally?

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

New function, see #102

What is the current behavior? (You can also link to an open issue here)

N/A

What is the new behavior (if this is a feature change)?

N/A

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

Other relevant information:

Thank you for taking the time to submit this PR!

[mcguinlu]

This is fantastic - thanks so much @rdboyes! I've merged this into a development branch for the rob_blobbogram() function, as it'll need a little bit more work before I pull it into the main branch (allow it to work with multiple risk-of-bias templates rather than just RoB 2.0, allow for subgrouping by any variable not just by risk of bias, add tests to ensure it is robust) - but an amazing starter for 10!

Very happy to have your input on any of additional work needed to make this a more general purpose function - I'll draw the outstanding items up in their own issue and flag it for you in case you want to take a look!

Cross-ref to #102