Assert packages, fixes #49

[ijlyttle]

A new function, assert_packages(), can be used in place of code suggested by R Packages.

Instead of this:

if (!requireNamespace("pkg", quietly = TRUE)) {
  stop("Package \"pkg\" needed for this function to work. Please install it.", call. = FALSE)
}

You can use this:

assertthat::assert_packages("pkg")

---

This PR fixes #49:

• adds assert_packages() to new file R/assert-packages.R
• adds tests in new file tests/testthat/test-packages/R
• updates Roxygen to 6.1.1, roxygenizes package
• adds item to NEWS