Incorporate `stack` buffer into `walkers`

[shufay]

In the thermal code, the stack buffer is separate from the walkers buffer for MPI. The former should be concatenated to the latter such that only the walkers buffer is passed around in the population control algorithms. See for instance:

ipie/ipie/addons/thermal/walkers/pop_controller.py

Line 125 in 366c499

stack_buff = walkers.stack[iw].get_buffer()

ipie/ipie/addons/thermal/walkers/pop_controller.py

Line 165 in 366c499

walkers.stack[iw].set_buffer(buff[walkers.buff_size:])

ipie/ipie/addons/thermal/walkers/pop_controller.py

Line 245 in 366c499

buffer = numpy.concatenate((walkers.walker_buffer, walkers.stack[0].stack_buffer))

ipie/ipie/addons/thermal/walkers/pop_controller.py

Line 385 in 366c499

buffer = numpy.concatenate((walkers.walker_buffer, walkers.stack[0].stack_buffer))