Phase stepping implemented, not tested on hardware

[Fred Proctor <frederick.proctor-at-nist.gov>]

Stepper motor users,

I put in code for doing phase stepping, as an alternative to
step-and-direction. The code's on SourceForge, but not in a .tgz release
yet.

To specify this, just put this in your .ini file:

[EMCMOT]
EMCMOT = freqmod.o
STEPPING_TYPE = 1

It only works for freqmod.o (steppermod.o will come once this is
debugged). If you say 0, or leave it out, it uses step-and-direction.

The wiring looks like this, from a comment in emcmot.c:

  Phases are labeled p13 and p19, from Bridgeport stepper motor driver
  pins 13 and 19. The values are converted from step/direction using the
  following table:

  dir   p13  p19

  |  ^   0    0
  |  |   1    0
  +  -   1    1
  |  |   0    1
  v  |   0    0

  The new value of p13 is the logical result of (p19 != dir), and
  the new value of p19 is the logical result of (p13 == dir), where
  + dir is 1 and - dir is 0.

  The bits are "wired" onto the parallel port like this:

  MSB                      low byte                             LSB
  -----------------------------------------------------------------
  | p13-3 | p19-3 | p13-2 | p19-2 | p13-1 | p19-1 | p13-0 | p19-0 |
  -----------------------------------------------------------------

  MSB                       hi byte                             LSB
  -----------------------------------------------------------------
  |   x   |   x   |   x   |   x   | p13-5 | p19-5 | p13-4 | p19-4 |
  -----------------------------------------------------------------

  where p19-0 is pin 19 of axis 0, p13-5 is pin 13 of axis 5, etc.

Brave testers?

--Fred