Modified the 3D reconstruction based on conjugate gradient to
correctly perform displaced detector weighting
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.
Merge pull request #5 from Hakkk2002/develop

Reduce memory used by ProjectionsReader
Modified all remaining iterative reconstruction algorithms to use
the displaced detector filter in the appropriate way. Updated the
documentation
Merge branch 'master' of github.com:SimonRit/RTK