Useful enhancement to plotContours and plotDiffContours

Created by: rmhirsch49

Laura: this is something that has come up in my interactions with some users and I share their concern. Right now, the y-axis of these two kinds of contour plots are constrained to start and end with discharge values such as 1, 2, 5, 10, 20, 50,... There are times when it would be nice to be able to pick any arbitrary value for the lower or upper bound on the axis. Sometimes when the selected limits are too wide apart, part of the figure plots as white space (because there are no data in that region in the surfaces object). That would be nice to avoid. But also, sometimes we have situations where perhaps a maximum of 20 cuts off too much of the relevant high discharge space, but 50 goes to far and shows results that are very unlikely and thus not very meaningful and the user may want to set the limit at say 30 or 40. I would still like to keep the tick mark values simple (1, 2, 5, 10, etc.) but allow it to go to any value. This is similar to a change you made a while back in plotQTimeDaily so that the lower bound could be at any value selected. Thanks. Bob