Reduce memory used by ProjectionsReader

After raw data are converted to attenuation, the raw data are not needed
anymore in the mini-pipeline and can be released. This can save memory
especially when --lowmem is not used in reconstruction.

Discussion: this may cause multiple unnecessary deallocation/allocation if
--lowmem is used. Maybe it is better to let SetProjectionsReaderFromGgo
determine whether to set the flag based on whether --lowmem exists?