AP_Logger: add Write_NamedValueFloat

[peterbarker]

for getting diagnostics into the code during ddebug fast

This doesn't use any flash until you use it:

Board               AP_Periph  blimp  bootloader  copter  heli  plane  rover  sub
Durandal                       *      *           *       *     *      *      *
Hitec-Airspeed      *
KakuteH7-bdshot                *      *           *       *     *      *      *
MatekF405                      *      *           *       *     *      *      *
Pixhawk1-1M-bdshot             *                  *       *     *      *      *
f103-QiotekPeriph   *
f303-Universal      *