Raytracing

The code is getting some form of basic documentation. It looks like an 80s web page, but it is *something*.

1.0.7 was released too quickly: typos in code were fixed, including one in 1.0.7 that led to a syntax error.

Slightly better display with the comments (i.e. dual graph) are used.

Finite length elements can block a ray at the beginning, at the end or anywhere in between. Prior calculations assumed the ray was blocked after the element, which is not correct for (some) finite length elements. This has been fixed.

This simple update allows the use of comments in a display() to comment and show the code.

Calculations have been improved for field stop and field of view, increasing the speed and precision with a better algorithm (see in abcd.py the method fieldStop() and fieldOfView()). Also, vertices are tracked, so when can calculate BFL and FFL (that is, the distance between the focal spot and the interface at the front or back).