printing: MatPow should have Mul precedence
OctaveCodePrinter: indenter

Tests not updated yet.
StrPrinter: add kwarg to _print_Pow and _print_Mul to override symbol

Subclasses or others might wish to change `**` to `^` for example.
OctaveCodePrinter: Use ".^" for Pow and "^" for MatPow

This is overly simplified, might revisit for things like pi**2 later.
OctaveCodePrinter: identity matrix to "eye"
OctaveCodePrinter: tests for MatrixSymbols
OctaveCodePrinter: update tests for Matrix and symbol changes

Last few commits have their changes to tests all jumbled up here.
OctaveCodePrinter: minor cosmetic and comment changes