Monotonicity API is not consistent

[fred3m]

In setting up the regression tests I was attempting to set the default values for monotonicity from "flat" to "angle" and min_gradient=0.0 (which was the default up until May, except for during initialization). But I noticed that the value passed to the source initialization is not respected by the code, so somewhere there is a break in the argument chain where the user defined value is not making it all the way through to the operator. I was going to fix it as part of the regression testing ticket (DM-25060) but that ticket has gone on long enough, and I'd like to keep the PR simple, so I'm opening this issue to remind us to track down this inconsistency and (preferably) set the default to min_grad=0.0 and monotonicity="angle".