Three new OPL problems and one bug fix

[ChenfeiP]

Feel free to review the revised version of S06.03.VolumesByShells.PTP08.pg

[ChenfeiP]

Please consider these three problems for addition to the OPL. All three problems are scaffolded and require students to apply limit laws, derivative rules or trig identities one step at a time and identify the correct explanation for their step. This ensures students learn standard names for derivative rules/limit laws and correct terminology, as well as ensures they are able to break things into single steps. At present, there do not seem to be any such scaffolded problems in the OPL.

[drdrew42]

Thanks for these contributions!

There are a few things that need to be resolved here before we can merge.

First, you're adding a new university folder to Contrib, welcome! Please include a README.md in the new 'UMN-Morris' folder containing the github usernames (or name + email) of any individuals from your institution who should be allowed to push new problems into your university folder. Feel free to take a look at other university folders in Contrib for examples.

Next, the three new problems that you've added look good -- except for a few warnings generated by the limits problem. In part three, you have multiple answer blanks that specify x as the correct answer. This should, at the very least, be a string "x" (rather than bareword x), and preferably an explicit MathObject Formula("x").

Finally, there is a merge conflict with your updates to the Rochester washer/shell integration problem. It seems that this problem has already been updated to use PGML.

Changes Requested

• add Contrib/UMN-Morris/README.md with the names of approved contributors
• update the limit problem to resolve the bareword 'x' warnings
• remove the changes to OpenProblemLibrary/Rochester/setIntegrals20Volume/S06.03.VolumesByShells.PTP08.pg