Better documentation for initializing a beam, links from object docs to tutorials

[bhazelton]

It's hard to initialize a beam from scratch right now, not enough description about how to start and then to run the check method. It's also annoying to have to set the axes length parameters in addition to setting the arrays. Maybe a easy_init class method where you pass the minimal set of info and it defaults all the rest sensibly?

Also, the write-ups we have are on the tutorials, need links to them from the object API docs.

interpolation method is not set by default but there's really only one option for each beam type. This should at least be made clear in the initialize description, could consider defaulting.