Clean up `importer.py`

[StokesMIDE]

importer.py is one of the first modules written, and as such, it has various gnarly and/or vestigial bits that add unneeded complexity to the API. There are several things in it that could/should be cleaned up; since none are significant enough to merit its own issue, they are collected here.

• The 'updater' system is rough and should probably be redone.
• Get rid of the updater-related extra arguments passed to functions that use it (e.g. readData()); if numUpdates and the like are needed, put them in the updater.
• Sensor defaults and parser types don't need to be arguments; this flexibility was never used and just clutters things. These can just use the module variables directly.
• Consider isolating all the legacy stuff; it has been years since IDEs didn't contain RecordingProperties self-description data.
• Have some of the keyword arguments default to None instead of objects or collections. For example, the user doesn't care that the default for updater is nullUpdater; that's just an internal detail and should be sorted out internally. This will make the signatures and help text more readable.

This issue will be updated in the future.