Confirm the compatibility between C++ OpenIGTLink and pyigtl

[brad-t-moore]

A write-up confirming the relationship between the 4x4 matrix used in pyigtl and OpenIGTLink and ITK image metadata. In seems that pygitl doesn't encode the spacing and direction directly (inferring it from the 4x4 matrix) but then parts of the OpenIGTLink standard suggest these are separate fields. Recommend examining the OpenIGTLink C++ implementation and the pygitl implementation to confirm.