Improve error propagation when stellar parameters are computed from photometry

Instead of using linear Gaussian error propagation, uncertainties for
radius, mass, and luminosity are now computed using Gaussian error
propagation based on a Monte Carlo method. While this yields basically
the same results as the linear approach for small errors, it is much
more reliable for large uncertainties. In particular, it will no longer
yield negative radii, masses, or luminosities.