listmode reconstruction and time frame: missing set functions

[KrisThielemans]

It makes sense to be able to specify the time frame to reconstruct. It looks like the code will take time frame definitions into account (although I have never tested this). It works by setting time frame definitions (defaulting to "empty") and current_frame_num (defaulting to 1). The frame definitions and frame number can be parsed. However, we have no set_time_frame_definitions, nor set_current_frame_num.

The frame_defs member is publically accessible though (amazingly). So, it can be set.

Nevertheless, we need to provide set functions, and make frame_defs protected (I guess for STIR 7.0).