Class and file organization for PPoint and PLine

[chinmay]

Because of the duality between points and lines in the projective plane, most of the data and code we use to represent these concepts in C++ are the same. If at all possible, we should avoid writing the same code twice for two reasons. First, the initial work in creating the programs is doubled. More important, maintaining the code is also made more difficult; if a change is required to the code, we need to remember to make that change twice. When we are fussing with our programs and making a number of minor modifications, it is easy to forget to update both versions. To illustrate this, we deliberately include a subtle flaw in the first version of the program; we then repair the problem once (not twice).