babl: rename functions that used to refer to specific gamma

babl: rename functions that used to refer to specific gamma

The definitions of components and formats for linear and nonlinear formats in
model-rgb and model-gray are now using the TRC of a BablSpace, thus in the
format syntax ' now means nonlinear, not sRGB / 2.2 gamma - even though it ends
up meaning that for formats create without specifying the space.