Allow Steps to export explicit chain of custody data at runtime

[pydsigner]

The way the Step API is designed, Steps receive one input path and a variable number of output paths, already identified at the beginning of each processing cycle. This is generally effective, but the edges have been pushed really from the beginning with JinjaMarkdownStep, which reads the input markdown file but also a Jinja template referenced by the markdown file. More recently, ResourcePackerStep completely does away with the input file meaning anything at all except for gathering other files. This all works well enough, but as we look to features like #30, which will require knowing all files that could affect the output of the step, we see that the engine's knowledge is insufficient. Further, proposed functionalities like #27, #28, #34, and #35 are all really going to require output paths calculated by the Steps themselves at runtime. #29 offers a possible workaround, but leans towards duplication of effort. Instead, let's take advantage of the fact we don't return anything from Steps currently. Step.__call__() can be extended to allow either the current None, in which case custody will be handled as at present, or a tuple of two lists of Paths, the first being input and the second being output. Steps which wish to declare only one of the two can easily return the paths that go in as input; this will make Step.__call__() totally symmetrical apart from the input path parameter being singular and the input path return value being plural.