SystemAssembler.init_global_tensor no longer wrapped

[Patrick Farrell]

SystemAssembler.init_global_tensor is crucial for users who want to interface to solver libraries (e.g. PETSc) themselves.

It used to be exposed in the SWIG interface, but is no longer available since SWIG was removed.

[Prof Garth Wells]

@pefarrell Good chance for you to learn how to develop with pybind11 - PR welcome!