remove pyramid_on_features from Menpo

`pyramid_on_features` was a redundant kwarg - in order to set it `True`, one had to supply a single callable to `feature` in place of a list of callables. A list was only allowed if `pyramid_on_features` was `False`.

Instead, this PR drops all that, and makes a simple rule:

1. If a single `features` is present, then we have the `pyramid_on_features=True` behavior
2. If a list of `features` present the list must be `n_levels` long, and we have the `pyramid_on_features=False` behaviour.

This is on top of #443.
If `pyramid_on_features` is false we are falling back to the default feature. We may want to think about what that default is in light of this change. As this is SDM it sounds like we are fine here - default is a no_op so we don't actually care, right?