interp/python: make EmcPose and PmCartesian instantiable

some canon calls like USE_TOO_LENGTH_OFFSET() or SET_TOOL_TABLE_ENTRY()
take an EmcPose as parameter. Add zero() method to clear.

Without this, only values from interp can be passed, which is silly.
import emccanon

oword.py example:

from interpreter import *

def set_tlo_x(self, *args):
    if len(args) != 1:
        return "set_tlo_x takes exactly one parameter"
    p = EmcPose()
    p.zero()
    p.tran.x = args[0]
    emccanon.USE_TOOL_LENGTH_OFFSET(p)