Need better way to modify experiment design

[hsharrison]

I forgot a trial in a design matrix, and forgot to increase the number of participants, before creating an experiment. It was more difficult than I'd like to make changes. Re-shuffling after appending a trial, required me to access the "private" method ExperimentSection._children.

We should have:

• better low-level APIs to supplement append_child, such as a method to re-order the children.
• a high-level API to re-load an experiment from a modified spec (e.g., YAML file), making changes where it doesn't impact already collected data and otherwise perhaps giving a warning or presenting the user with choices.