[Feature] Refactor so THRESHOLD and SOMA_CENTRE_VALUE are defined in one place

[alessandrofelder]

Is your feature request related to a problem? Please describe.
In cell candidate detection, we set aside two very high integer values (currently SOMA_CENTRE_VALUE as np.uint16.max-1 and THRESHOLD as np.uint16.max-2) to mark pixels as belonging to a cell after 3D and 2D filtering respectively.

This is a problem (see #383), so will switch to np.uint32.

Additionally though, we define this in several places

• cellfinder/cellfinder/core/detect/filters/volume/structure_detection.py

  Line 164 in 749af84

  SOMA_CENTRE_VALUE = np.iinfo(plane.dtype).max

• cellfinder/cellfinder/core/detect/filters/setup_filters.py

  Line 24 in 749af84

  max_value = get_max_possible_value(plane)

• cellfinder/cellfinder/core/detect/filters/volume/structure_splitting.py

  Line 95 in 749af84

  threshold_value = 65534

Describe the solution you'd like
These important labels should only be defined in one place.