bt2: _IntegerRangeSet.add(): make it possible to add a single value

bt2: _IntegerRangeSet.add(): make it possible to add a single value

This patch makes it possible to do:

    my_range_set.add(23)

which is the equivalent of:

    my_range_set.add((23, 23))

It also makes it possible to do:

    bt2.SignedIntegerRangeSet(-17)
    bt2.UnsignedIntegerRangeSet(17)

which is the equivalent of:

    bt2.SignedIntegerRangeSet(-17, -17)
    bt2.UnsignedIntegerRangeSet(17, 17)

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I44bf1a257422fb9e74835a300eeda948a8e5d067
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1874
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>