Add half- and double-bandwidth estimates for nonparametric frontier approach (Table 3.1 in shiny)

[papsti]

Currently, I'm only working on displaying nonparametric estimates for the frontier approach with the optimal bandwidth, similar to how we've made the choice to only display the linear parametric estimates for the frontier design. The latter choice is due to the fact that the quadratic and cubic regressions aren't implemented for frontier, but in the non-parametric case, we do have the half- and double-bandwidth models implemented, so we could conceivably display them. The only issue is that it may make Table 3.1 very cluttered.

Is it important to display the estimates for the half- and double-bandwidth to shiny users?