Export density* functions as S3 methods of dtrunc

[wleoncio]

This would require changing the class of the inputs of those files (y and eta) so they can be properly matched by the generic density() function. Otherwise, the functions won't work as methods for density().

[wleoncio]

After meeting, I understood the purpose of the density.* functions in this package a bit better. They should be methods of a generic dtrunc() function (to be created), drawing a parallel to the stats package's dnorm/rnorm functions. So this is unrelated to the generic density() functions.

[wleoncio]

@rho62, could you please check if the documentation on R/dtrunc.R describing the function is correct?