[graphs] Add Mathematica linear best-fit to demand curves.

I'm pretty sure this is what we are 'supposed' to be using,
because the coefficients are suspiciously close to rational
numbers.
[curve] Implement LinearCurve.
[world] Add the compile-time option to use linear interpolation of demand.
[opt] Improve error bounds for linear. Deal with negative MR in monopoly.
[opt] Only report at beginning and end.
[main] Add section headings.
[results] Add results with cubic spline and linear demand curves.