Progressions preface and introduction, and updates to Algebra and Functions

A a couple of things today. First, a a draft of the front matter for the Progressions, including an introduction explaining the sources of evidence, organization, and terminology for the standards. It also lists the members of the work team that produced the Progressions, who have been sadly unacknowledged until now. I would like in particular to call attention to the work of our editor, Cathy Kessel, who is also an occasional contributor to this blog.

Second, thanks to work of Al Cuoco, we have updated versions of the Algebra and Functions Progressions.