Estimation can fail

When the detector is not properly setup.

Traceback (most recent call last):
  File "/nobackup/lvalls1/valls/workspace/daiquiri-tomo.git/daiquiri.git/daiquiri/implementors/tomo/tilingscan.py", line 217, in time_estimate
    tomoconfig=tomo_config,
  File "/nobackup/lvalls1/valls/Software/miniconda3/envs/tomodemo-daiquiri-env/lib/python3.7/site-packages/typeguard/__init__.py", line 1015, in wrapper
    retval = func(*args, **kwargs)
  File "/nobackup/lvalls1/valls/workspace/daiquiri-tomo.git/bliss-tomo.git/tomo/sequence/tiling.py", line 104, in tiling_estimation_time
    xstart, xstop, lateral_motor_mode, tomoconfig, tomo_detector, real_motor=True
  File "/nobackup/lvalls1/valls/workspace/daiquiri-tomo.git/bliss-tomo.git/tomo/sequence/tiling.py", line 46, in _get_y_axis_info
    tomo_detector, ystart, ystop, lateral_motor_mode, real_motor=real_motor
  File "/nobackup/lvalls1/valls/workspace/daiquiri-tomo.git/bliss-tomo.git/tomo/scan/tiling_runner.py", line 157, in get_y_trajectory_info
    _, fov_width = self._get_fov_shape(tomo_detector)
  File "/nobackup/lvalls1/valls/workspace/daiquiri-tomo.git/bliss-tomo.git/tomo/scan/tiling_runner.py", line 133, in _get_fov_shape
    if pixel_size <= 0:
TypeError: '<=' not supported between instances of 'NoneType' and 'int'