isPackageLoaded: Checks if a package is loaded or not
Description
  Checks if a package is loaded or not.
  Note that, contrary to require(),
  this function does not load the package if not loaded.
Usage
"isPackageLoaded"(package, version=NULL, ...)
Arguments
package
The name of the package.
version
A character string specifying the version to test for.
    If NULL, any version is tested for.