Plus42 algebraic expressions update

[Thomas Okken]

PRINT is going to show a menu, with at least the options "print current equation," "print variables for current equation," "print all equations," PON, POFF, and, once I understand what they do on the 17B/27S and assuming they make sense here as well, MAN/NORM/TRACE/STRACE. I'll have to find another key for immediate "print current equation without going through the menu."