Updated description: This MR addresses the case of annotations missing for some frames. Basically now the preprocessor "has a chance" of addressing the case of missing frame annotations. Before the execution was failing without reaching the frame preprocessor.

An example of the class using an updated Wrapper can be found here: https://gitlab.idiap.ch/bob/bob.pad.face/blob/preproc_refactor/bob/pad/face/preprocessor/FaceCropAlign.py

A very trivial example can also be found in the unit tests.