Separate cache initialization from polling start/stop.

[jsoules]

Fix #17.

This PR separates the step of initializing the MCMCDataManager cache from the control for turning its polling off and on, which is now attached to the RunPage page/component directly so that the polling gets terminated in the cleanup step when we're on a page where it isn't relevant.

To do this, I've moved the cache object itself into the base App level and have the reference passed down a couple levels. It's not the greatest, but I didn't really want to set up yet another context object; however, it's possible this would be better managed by a custom hook. I played around with that for a bit but the underlying state wasn't updating properly, so I left it be and stuck with this solution, inelegant though it may be.

[jsoules]

To test:

• Added console logging messages on data manager initialization, iteration, starting, and stopping.
• Confirmed that initialization happens once (on page load).
• Confirmed that manager starts when navigating to a runPage and stops when navigating away.
• Confirmed that iteration messages are printed when manager is active (between start and stop messages) and not otherwise.
• Confirmed that UI continues to do updates as expected.

[magland]

This seems like a good solution to me.