[BUG] Possible wrong likelihoods in FBD range processes

[bjoelle]

Both FBD range processes dnFBDSP and dnFBDRP are possibly broken and should not be used at the moment
In particular it was reported to me that dnFBDSP shows problematic behaviour in real conditions - like origin time going to infinity
The branch rapture_FBD contains older, more stable versions of both these processes for anyone interested in experimenting - note that neither model (in either branch) handles fossil age uncertainty properly at the moment