Allow user to define start and stop angle + angle step between projection when convert it.

issue raised by @broche. Requested by several users.

Can be done during conversion or as tomwer widget / option to nabu ? Seems to be better done in nxtomomill.

In fact could be a subsection of an existing NXtomo. Would operate once the first NXtomo is created.

Like if we have acquisition which goes from 0 to 720 we would like to create one NXtomo starting at 45 to 225 then from 405 to 585 ...

In fact this would be as the current PCOTomoAcquisition but with a start_angle that could be defined by the user and a angle_range defined by the user instead of having those deduce from nb_tomo and nb_turn.

Question: what would be the test interface for it ? Let the user define a configuration file and edit it or provide some advance setting on then nxtomomill widget ?