Pybind rebase

This is a rebase of TJ's pybind branch that adds pybind11 source files along with a cmake option (SMTK_USE_PYBIND11) to build python bindings with pybind11 instead of Shiboken. (By default, the standard Shiboken-generated bindings are built.) Additional development is still needed before using the pybind11 option with CMB, however, this branch is sufficient for testing standalone SMTK builds.