This guarantees that BasePlugin instances have the attributes set there
without having to assume anything about their subclasses.

This removes some code duplication at the expense of a bit of opaque
argument-forwarding.  It'll become more important shortly, as the
now-shared code is about to grow.

This changes aperture flux outputs to look more like an array-of-structs
rather than a struct of array, e.g. "base_CircularApertureFlux_[0,1,2]_flux"
instead of "base_CircularApertureFlux_flux_[0,1,2]".  This makes it
its field suffixes compatible with the slot mechanism.

Now that it can participate in slots, CircularApertureFlux can do everything
NaiveFlux and SincFlux once did, so there's no need for all that code duplication
and boilerplate.

A "using" declaration to un-shadow the multiple overloads of measure() doesn't
appear to work with Swig, so we move those methods to protected to make
Swig continue to call throught the base class interface, which works fine.