enable constraints on parameter ranges

[jbloom]

Enable setting constraints on parameters (eg, top and slope) that are reasonable ranges rather than fixed values.

This is now feasible with neutcurve 2.0.0 (see this issue).

Probably in general we want to constrain the top to something in the range of 0.7 or 0.8 to 1, and we probably want to constrain the slope to some reasonable range that is not step-like step or very shallow.

In particular, this should help deal with curves that are always or never neutralized, as well as ones with plateaus less than one.

This needs to be implemented into the pipeline.

[jbloom]

#34 updates neutcurve, still need to put range constraints into pipeline.