Implement integration with knitr (draw.*)

[fabrice-rossi]

The current version of draw.* function (improved in issue #66) can produce latex export, but in a rmarkdown document or a quarto document, we need to include the adapted latex style via the header and to specify that the chunk result as to be 'asis'.
This is a bit cumbersome while a custom print for knitr should remove both problems. We need

• to modify draw.* functions so that they output a mixvlmc_draw object
• to implement print.mixvlmc_draw() to trigger standard printing when needed
• to implement knit_print.mixvlmc_draw for automatic knitr printing
• to register the latex dependency with rmarkdown::latex_dependency() and register them with knitr::knit_meta_add()

[fabrice-rossi]

After further thoughts, it seems better to remove draw functions and replace them by extraction functions that produces an object with adapted printing methods. This separates the drawing action from the printing part, as in the xtable package for instance.