Also added a couple of missed merge lines in homeaxis() from previous
commit.

add more delta defaults
on delta make second home even slower

Dual x-carriage designs offer some substantial improvements for dual
extruder printing.

M109 S180 waits only when heating.
M109 R180 also waits when cooling.

Diffed and merged, preserving my updates

Also refactor extruder auto fan logic to simplify further and now only
check every 2.5 seconds

This change allows fan outputs to automatically turn on/off when the
associated nozzle temperature of an extruder is above/below a threshold
temperature.
Multiple extruders can be assigned to the same pin in which case the fan
will turn on when any selected extruder is above the threshold.
It also makes the M42 command compatible with the M106/M107 command.
The majority of the logic in this change will be evaluated by the
compiler at build time (i.e, low code space requirements).

[default off for now]
syntax: M600  X[pos] Y[pos] Z[relative lift] E[initial retract] L[later retract distance for removal]

if enabled, after a M600, the printer will retract by E, lift by Z, move to XY, retract even more filament.
Oh, and it will display "remove filament" and beep like crazy.

You are then supposed to insert a new filament (other color, e.g.) and click the display to continue.
After having the nozzle cleaned manually, aided by the disabled e-steppers.
After clicking, the printer will then go back the whole shebang, and continue printing with a fancy new color.

Add a feature to run the cooling fan at full speed for a small period
(default 100ms) when first starting the fan.  Some fans wont reliably
start spinning at low power, and many fans have issue with the PWM at
low power.  However, once the fan starts spinning it can reliably be
set to a wide range of PWM values.

fixes #246
Added statements to set the limit switch positions to the maximum travel if homing in the positive direction as well as bed center at (0,0) if defined.
Relocated code based on feedback.

motor current and microstepping pins.

Changed SLOWDOWN. IF this does not work ok OLD_SLOWDOWN is the old algo.

Fixed small bug in arcs
M105 prints now actual_temp/target_temp

One array was too short. This had nothing to do with long filenames, other than if they were 12 characters exactly, which could only happen if the extension and the text before were filled completely

Included AD595 calibration options. (daid)

Instead of a single pre-heat, now there is pre-heat ABS and PLA options
Added defines to the configuration file to adjust preheat temperatures for both

Simplified stepper inactive time

Simplified stepper inactive time

One for common settings.
One for advanced settings.