WIP: read_retries and trig_readout_keep_first_point

P201 @alejandro.homs modifications 2018 on id21 to keep first point and retry on DMA access.

• retries 5 times reading of point in case of error
• do not remove first point
• taken from LDCM1
• doc: properties skeleton
• doc: classes diagram

@sebastien.petitdemange : any comment on compatibility with your modifications ?