moved ordered slotted-positional sequences to ordered groups module

Signed-off-by: greenTara <taraathan@gmail.com>
modify RNC to allow Interval to be an empty element

Signed-off-by: greenTara <taraathan@gmail.com>
adding example file with @per="effect", for partial resolution of Issue #43

Signed-off-by: greenTara <taraathan@gmail.com>