[REF] Flip the bits on the histological profiles

[ReinderVosDeWael]

For the histological module "The Merker staining labels cell bodies, similar to Nissl staining, with a high contrast between black cell bodies on a light background (Merker, 1983). In the digitised images, darker colouring is represented by lower numbers (8bit graphics: 0–28 = black-white). It is common practice to invert the values of the intensity, such that image intensity increases with staining intensity." Paquola et al., 2021, eLife

Currently, these values are not inverted in BrainStat, we should add this as a default.

Note that this should not affect MPC computations.