Allow vector ids in calls

[martigso]

One recurring issue for users is the (slightly) more complex iterative reliant nature of the package. Currently, all functions are built in a way that for-loops or apply-family functions can be used to access multiple IDs. For instance, to download the MPs "JGS", "TSG", and "MOM", we currently need to iterate over these:

mps <- list()

for(i in c("JGS", "TSG", "MOM")) {
  mps[[i]] <- get_mp(i, good_manners = 2)
}

A more smooth user experience would be to allow the users to just supply the vector of IDs directly:

mps <- get_mp(c("JGS", "TSG", "MOM"), good_manners = 2)

In order to not break backwards-compatability, we could implement a vector length check, for instance.