simplify extracting resolution matrix from simulator

[sbailey]

The high-resolution "resolution matrix" that convolves the source spectrum by the instrumental resolution is available as qsim.cameras[j].sparseKernel. It would be handy if the user could access the binned equivalent after downsampling, i.e. the resolution matrix corresponding to the output flux. I think we would want it to have the property that

result = qsim.simulate(..., downsampling=n)
R = qsim.get_binned_resolution(..., downsampling=n)
np.all(result.obsflux == R.dot(result.srcflux))

Or at least as close to that as is possible given the rebinning.

[dkirkby]

Fixed by #35