BM23: execute example_bm23 when saved before

Traceback (most recent call last):
  File "/users/blissadm/conda/miniconda/envs/ewoksworker/lib/python3.7/site-packages/ewokscore/task.py", line 337, in execute
    self.run()
  File "/users/blissadm/local/est/est/core/process/noise.py", line 109, in run
    self._pool_process(xas_obj=_xas_obj)
  File "/users/blissadm/local/est/est/core/process/noise.py", line 131, in _pool_process
    overwrite=True,
  File "/users/blissadm/local/est/est/core/process/noise.py", line 71, in process_noise_savgol
    spectrum.norm_noise_savgol = numpy.mean(noise) / spectrum.edge_step
TypeError: unsupported operand type(s) for /: 'float' and 'NoneType'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/users/blissadm/conda/miniconda/envs/ewoksworker/lib/python3.7/site-packages/ewoksorange/bindings/taskexecutor.py", line 31, in execute_task
    self.__task.execute()
  File "/users/blissadm/conda/miniconda/envs/ewoksworker/lib/python3.7/site-packages/ewokscore/task.py", line 343, in execute
    raise RuntimeError(f"Task '{self.label}' failed") from e
RuntimeError: Task 'noise' failed