Move BasePlugin, BasePluginConfig, BaseMeasurementTask and
BaseMeasurementConfig to a new module baseMeasurement.py
in order to allow BaseMeasurementTask access to ApplyApCorrTask.
Without this refactoring it would be very difficult to avoid a
circular import. This change is arguably an improvement because it
moves higher-level code out of base.py and moves BaseMeasurementTask
into a more obvious module (one that analogous to sfm.py and
forcedMeasurement.py)

BaseMeasurementTask and its subtasks now apply aperture correction
as part of the run method. Most of the work is done in
BaseMeaurementTask._applyApCorrIfWanted, a simple private wrapper
around self.apCorrMap.run that only applies aperture corrections
if wanted.

- Eliminate use of "from X import *"
- Remove unused imports
- Add a missing math import in one test
- Add __all__ to some modules to avoid importing too many symbols
- Import a few more files in __init__ (whose symbols were being
  pulled in due to lack of "__all__" in other modules).
- Reorder a few imports to meet LSST standards
- Stop assigning to variables that are not used; this showed
  up in a few unit tests and I left the computation in place,
  since performing the computation looked like it would
  probably be a useful test.