added new helper function to geometry.js

I’ve added a function to geometry.js, lineEq1, which takes two points (a,b) and (c,d) and returns the LaTeX form of the equation ax+by+c=0 (note the reversed x and y).

If you have an equation in the form y=mx+c, you can get the other form by taking x=0 and x=1.
added dependency for helpers/geometry.js
added mathmo question for finding line parallel/perpendicular to a given point