New commands for rendering the equation of a line ...

\WRITELINEEQUALSZERO writes the equation in ax + by + c = 0 form.
\WRITELINEMXPLUSC writes the same in y = mx + C form.

The former uses \WRITEPOLYTERM. Hence \WRITEPOLYTERM moved to g-2d.sty.
Merge pull request #859 from abhinavc/master

\WRITELINEEQUALSZERO and \WRITELINEMXPLUSC defined ...