[Implemented] Should {tlf} be a hard or a soft dependency?

[IndrajeetPatil]

@Yuri05 had mentioned the following:

@IndrajeetPatil @PavelBal @msevestre
Do we want to have tlf and patchwork as mandatory prerequisites (like now) or is it sufficient to have them as optional prereqs?
I would vote for optional, because - except for plots - ospsuite is still fully functional without those packages installed.

Originally posted by @Yuri05 in #882 (comment)

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1. This would mean that none of the functions from {tlf} can be in the ospsuite NAMESPACE and the plot grid functions/classes can't be re-exported, like so:

#' @export
#' @importFrom tlf plotGrid
tlf::plotGrid

#' @export
#' @importFrom tlf PlotGridConfiguration
tlf::PlotGridConfiguration

2. This also increases the code complexity because we will need to check if tlf package is installed at the top level every time tlf is used in code, examples, tests, vignettes, etc.

Just wanted to make sure that the team was on-board with this before I made the necessary changes.

[PavelBal]

Actually I am for mandatory. I would hate it to suddenly realize that some functionality of the package does not work because a dependency is optional.

[Yuri05]

ok, let's make it mandatory then