OctaveCodePrinter: minor cosmetic, add comments
OctaveCodePrinter: add booleans
OctaveCodePrinter: add extra parentheses to PieceWise for 2*pw

Unnecessary in some common cases.  Later could try to do this properly
with precedence.parenthesize().
OctaveCodePrinter: Matrix elements printed correctly (oct array is 1-based)
OctaveCodePrinter: add draft for docstring