FBP improvements

This MR brings some improvements to the filtered backprojection (FBP) module.

Close #275 (closed), #276 (closed)

To do

• Rework Backprojector API
• Adapt unit tests and pipeline to this new API
• Adapt OpenCLBackprojector API
• Add "clip outer circle" (PyHST "ZEROOFFMASK")
• Unit test for "clip outer circle"
• Add "centered axis (PyHST "axis_to_the_center")
• Unit test for "centered axis"
• End-to-end tests