Add new functions for the root mean square of a (Calc) vector

Add new functions for the root mean square of a (Calc) vector

* lisp/calc/calc-stats.el (calcFunc-rms, calc-vector-rms):
New functions.
* lisp/calc/calc-ext.el (calc-init-extensions): Add keybinding for
`calc-vector-rms', add autoloads for `calc-vector-rms' and
`calcFunc-rms'.
* lisp/calc/calc-map.el (calc-u-oper-keys):  Add entry for
`calcFunc-rms'.
* lisp/calc/calc-menu.el (calc-vectors-menu): Add entry for
`calc-vector-rms'.
* doc/misc/calc.texi (Single-Variable Statistics): Document the rms
command.