Check for package dependencies in `new_sim`

[Avi-Kenny]

We formerly had a section that looked like this, but removed it because of bugs:

  # Check if dependencies are installed
  for (pkg in tools::package_dependencies(packages="SimEngine")$SimEngine) {
    if (!requireNamespace(pkg, quietly=TRUE)) {
      stop(paste0(
        "You need to install the package '", pkg, "' for SimEngine to work."
      ))
    }
  }
  rm(pkg)