Estimate time for path

[fragmuffin]

Add functionality to pygcode.Machine to estimate time needed to follow a CNC path.

For example:

>>> m = pygcode.Machine()

# Initial state
>>> m.pos
<Position: X0 Y0 Z0>
>>> m.time
0.0

# After movement
>>> rapid_move = pygcode.GCodeRapidMove(X=10, Y=20)
>>> m.process_gcodes(rapid_move)
>>> m.pos
<Position: X10 Y20 Z0>
>>> m.time
1.23

Scope: (estimate only)
In scope:

• Delays from acceleration / deceleration to varied speeds
• Configured jogging speed (of pygcode.GCodeRapidMove)
• Feed-rate of pygcode.GCodeMotion.__subclasses__()

Out of scope:

• micro-arcs injected during instant direction-changes
  eg G01 X0 Y10 -> G01 X10 Y10, tool can't instantly change direction at full speed
  For the purposes of this estimate the above example would take the same time as G01 X0 Y20