[FEATURE] modality-specific minima definitions for BET and defacing

[neuronflow]

• define separate background values for BET and defacing
• adjust HD-BET and Synthstrip masking and output generation
• rewrite defacing apply mask logic
• rewrite how skullstrippers mask the center modality
• do not forget about TODO comment in defacer docstring :)

[neuronflow]

this will also affect:
#171