Added support for ReadoutMode
Merge branch 'multi_frame_read'
* Formalized core feature control in LimaCCDs for backward compatibility
* Changed DATA_ARRAY header definition (version 2):
  + Max number of dimensions is reduced to 6
  + DimStep is now a 32-bit arr. with the dimension strides in bytes:
    + By default DimStep[0] is the size of the data
  + The total header length is unchanged, v2 is backward compatible if
    NbDim <= 6 and DimStep is not used.
* Limit the event push rate to a maximum value, configurable
  by the image_events_max_rate attribute (default 25/s)
* Renamed last_image_pushes_events attr. to image_events_push_data.