create_pyramid now applies features even if pyramid_on_features=False, SDM uses it too

Previously, `create_pyramid` created a pyramid that was ready to use if `pyramid_on_features=True` (i.e. it applied the feature for us), but if `pyramid_on_features=False` we have to remember to apply the features ourselves. This lead to repeated code in the AAM and CLM build methods.

This PR changes this so that the `create_pyramid` yields a pyramid of feature images in both cases, so there is no need to do the extra logic in the build methods. It also removes the repeated `@classmethod`s from SDM for doing the same job - now all deformable models use the same pyramid of features creation routine.

Note that this does not change the order of the feature computation, the pyramid generation, or the use of the `pyramid_on_features`, so it should be functionally identical to master.