This function uses Java's Runtime class to run the garbage collector.
Java memory is freed by first running R's garbage collector (gc) and then
Java's garbage collector. This sequence is important as R's gc may release
objects which in turn allows Java's garbage collector to release some objects.
Note, in general there should be no need to make active use of this with XLConnect. Both
R and Java automatically perform garbage collection at times. However, this function might be useful
to reclaim memory after removing a large data object that has been written/read to/from Excel.