add flux.kron.psfRadius to record radius of PSF

Want to record the PSF radius, as that can affect the radius chosen
for the aperture.
Move minimum radius logic to KronFlux::_apply

_applyAperture should just use the aperture we've already decided on, not
modify the aperture.  This ensures the apertures in the different bands
are identical (previously, they could vary according to the seeing in
each band, if the original radius came back small).
make KronAperture::_axes mutable

We manipulate the size of the aperture (e.g., resizing for minimum radius),
but because KronAperture::getAxes returned by value, we were modifying a
copy and the change wasn't effective.  Have getAxes return by reference,
and add const and non-const versions so we can modify the aperture.
only set badRadius flag when we detect a bad radius

Previously, it might be set even if there was no bad radius,
but there was an exception.
correct calculation of Kron radius of PSF

When calculating the Kron radius of the PSF, the factor for converting
a Gaussian sigma to the Kron radius should only be applied to the
Gaussian sigma, not the Kron radius.
calculatePsfKronRadius: use PSF radius instead of calculating ourselves

RHL writes: """
I think we should just use the 2nd moment. The comment complains that we
can't use computeGaussianWidth, but it's been superseded by
computeShape().getDeterminantRadius() which is const-qualified. If we do
this the quadrature sum of the moment and smoothingSigma is correct, and
the sqrt(pi/2) should go outside the sqrt.
"""
Merge branch 'tickets/HSC-989'