Import truncdist properly

[jamesmbaazam]

I'm getting a missing truncdist dependency on this vignette (and some others)

Originally posted by @adamkucharski in #122 (comment)

This can be fixed by either adding truncdist to the "Imports" field in the DESCRIPTION file and/or adding appropriately using @importFrom tags where the package is used, i.e., simulate_tree_from_pop().

Note that, an incoming change to merge the functions might mean that truncdist stays in "Suggests" for the vignettes because it will no longer be required.