Use MFnMatrixData when storing/retriving MMatrix from data block

[kxl-adsk]

Matrix is stored as shared data, we have to use an appropriate function interface to allocate and maintain the lifetime of matrix objects when stored in the data block.

Added regression test to validate the fix and prevent regressions. Normally it would require a test only for the converter, but data block is not something we have easy access to from python.

[ppt-adsk]

Copy-paste damage in comment (cached playback is enabled in this test).

[cfmoore007]

I just compiled this against latest dev branch and it looks good!

After running the following code, I was able to get the UFE object to follow the locator :

Example file :
polyCube_custom_xformOp_matrix4d.zip

`
from mayaUsd.lib import proxyAccessor as pa
import maya.cmds as cmds

cmds.loadPlugin("mayaUsdPlugin")

selUfeObject = pa.getUfeSelection() # select /pCube1 in my simple box example
pa.createXformOps(selUfeObject)
dir(selUfeObject)
selUfeObject.nodeName()
selUfeObject.nodeType()
selUfeObject.path()
matrixPlug = pa.getOrCreateAccessPlug(selUfeObject, usdAttrName='xformOp:transform:offset')

nodeDagPath = cmds.ls('PolyCube_custom_xformOp_matrix4d_usdShape',l=True)[0]
cmds.currentTime(1)
cmds.getAttr('{}.{}'.format(nodeDagPath,matrixPlug))

cmds.spaceLocator()
srcNodeDagPath = cmds.ls(sl=True,l=True)[0]
cmds.setAttr('{}.ty'.format(srcNodeDagPath), 3)

cmds.connectAttr('{}.worldMatrix[0]'.format(srcNodeDagPath), '{}.{}'.format(nodeDagPath,matrixPlug)) # Connect src to drive matrix op
`

[cfmoore007]

Basic usability testing looks good.

Unit tests also pass here (build/test/lib) :

100% tests passed, 0 tests failed out of 79

Label Time Summary:
interactive = 35.77 secproc (3 tests)
pxrUsdMayaGL = 35.77 secproc (3 tests)
translators = 140.75 secproc (47 tests)
ufe = 56.44 secproc (20 tests)
usdPreviewSurface = 5.05 sec*proc (2 tests)

Total Test time (real) = 257.35 sec