BrainWeb-based multimodal models v1.6.0

• fix get_mmr_fromfile arguments being ignored (#5 <- #3)
• fix toPetMmr ignoring modes (#7)
• cache & make configurable: resolution & PET activity class (#8, #6 <- #4)
• add `get_label_probabilities() (#8 <- #2)
  • add Act.all_labels
  • expose Act
• add trim_zeros_ROI()
• make toPetMmr() return ndarray
• update example images
• update dependencies