change logical ordering parameters to character

[corybrunson]

The parameters decreasing, reverse, and absolute determine the order of the strata, lodes, flows, and alluvia at each axis. Currently, they take logical values, in the first case with a special meaning ascribed to NA. It might make code more intuitive and readable to replace these with one parameter that, if possible, takes accepts computed variables, e.g. order_by = desc(stratum) or order_by = desc(abs(n)).

I'm uncertain about this and welcome feedback.