Rebalance and rename model fit statistics

[bpbond]

This PR reorganizes, renames, and rebalances the model fit statistics used.

The columns now follow a standard naming format: {model type}_{statistic}:

Column	Description
HM81_AIC	H&M (1981) model AIC
HM81_p.value	Model p-value
HM81_r.squared	Model R squared
HM81_sigma	Model residual error
HM81_flux.estimate	Flux estimate
lin_AIC	Linear model AIC
lin_p.value	Linear model p-value
lin_r.squared
lin_sigma
lin_flux.estimate
lin_flux.std.error	Standard error of the flux estimate
lin_int.estimate	y-intercept
lin_int.std.error	Standard error of the intercept
poly_AIC	3-term polynomial model AIC
poly_r.squared
rob_AIC	Robust linear regression model AIC
rob_converged	Convergence TRUE or FALSE
rob_sigma
rob_flux.estimate
rob_flux.std.error

The new vignette section: