Updating polls

[agitter]

Some of the polls in the 2021-02-25 workshop were effective and others were not as effective. Specific suggestions include:

• Remove the first poll "T-Cell Classification: Exploring Model Training". There is not a clear answer, and the activity takes 5 min to complete.
• Double check which polls should be multiple choice.
• Add more frequent short polls throughout for formative assessments. For instance, short polls about loading a dataset and listing the instances and then listing the classes would reinforce those terms.

[agitter]

For the 2021-05-04 workshop I updated all the polls to match this version. That addresses the first unclear question and the single versus multiple choice issues.