mathmo questions on circle/line intersections

This pull request finishes the questions discussed in [issue 16](https://github.com/gmp26/Apps1/issues/16).

Specifically, it adds:

* A full question and answer for circle and line intersections, where the computer chooses between C/C, L/C and L/L
* Minor changes to the circle equations for prettier printing (736105b)
* A helper function for simplifying two part surds

Please also note the commit message for 27643e7 (specifically, an incorrect topicID which has now been fixed).