first proof of concept of MRAN repos, and some work-arounds

R-3.1.1
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For better reproducibility:
- Use a named debian version
- Use as only apt repository the debian snapshot
- default R repository set to a relevant MRAN snapshot

R-3.0.2
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- Use a named debian version
- Use as only apt repository the debian snapshot
-  R repository set to a relevant MRAN snapshot, but the problem is that the oldest MRAN snapshot is still too recent => many packages are too recent for R-3.0.2

R-2.12.0
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work-around: could not make it to work purely on a debian snapshot. Had to mix with the default repository using an apt preferences file to force only the r-* packages from the debian snapshot.