Changed default message behavior on switchTo calls

Commit b2bf2ad
This should allow for better integration with iohub.pandas.interestperiods that search for message events sent on ScreenState.switchTo().

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Commit ac1c1e6
You'll have to let me know how you feel about nested functions. I added `_extract_criteria_match` for the following reasons:
(1) it was code that was repeated for the start and end sources, and merging the start and end events together on the index should be safer than just stacking the additional columns onto self._ip_df
(2) draws focus to the functionality of `exact=True` versus `exact=False`
(3) this will hopefully allow for easier integration when we start to accept multiple criteria matches