add another method for deltaMethod()

[rpruim]

deltaMethod() is a generic function in the car package that does propagation of error by the delta method. I've written code that applies this to each row of a data frame for cases that arise often for engineers and physicists who take several measurements of similar things and have an uncertainty associated with each measurement (that is estimated from something other than the data itself).

This is a placeholder to remind me to add it to the mosaic package for ease of distribution to my students (and anyone else who wants to use it). I'm still working out exactly how I want the API to work. Once I get that figured out, I'll document it and get it into the package.

[rpruim]

I think I have a reasonable version of this. The interface allows for several ways of describing the necessary information. I'm pushing it to beta. I'll close this now and we can create new issues if we detect problems or want to add/change behaviors.

I'll be using this later this semester with my classes, so that will give it a good test.