[omtk2] Node Editor - Convert pymel logic to OpenMaya2

[renaudll]

Current (pymel) implementation is really slow when doing connection traversal.
I've done some quick benchmarks and OpenMaya2 seem the faster for this.

# Benchmark cmds vs OpenMaya vs OpenMaya2
# For each attr of each nodes, get the name of each connected attrs
import time
import pymel.core as pymel
from maya import OpenMaya
from maya.api import OpenMaya as om2

all_ = []
objs = pymel.ls()
print("Scanning {0} nodes".format(len(objs)))
st = time.time()
for obj in objs:
    for attr in obj.listAttr():
        all_.append(attr.name())
et = time.time()

print("Pymel took: {0}".format(et-st))

###

# Benchmark cmds vs OpenMaya vs OpenMaya2
# For each attr of each nodes, get the name of each connected attrs
import time

all_ = []
mfns = [obj.__apimfn__() for obj in pymel.ls()]
print("Scanning {0} nodes".format(len(mfns)))
st = time.time()
for mfn in mfns:
    for i in xrange(mfn.attributeCount()):
        a = mfn.attribute(i)
        amfn = OpenMaya.MFnAttribute(a)
        all_.append(amfn.name())
et = time.time()

print("OpenMaya took: {0}".format(et-st))

###

pymel.select(pymel.ls())
sel = om2.MGlobal.getActiveSelectionList()
all_ = []
print("Scanning {0} nodes".format(sel.length()))
st = time.time()
for i in xrange(sel.length()):
    mfn = om2.MFnDependencyNode(sel.getDependNode(i))
    for j in xrange(mfn.attributeCount()):
        a = mfn.attribute(j)
        amfn = om2.MFnAttribute(a)
        all_.append(amfn.name)
et = time.time()
print("OpenMaya2 took: {0}".format(et-st))

Result in:

Scanning 286 nodes
Pymel took: 6.77531504631
Scanning 286 nodes
OpenMaya took: 0.150294065475
Scanning 280 nodes
OpenMaya2 took: 0.108594894409