Thanks Jonas!
Fix gradient calculations for LinkNode and MixtureNode
Bump version number to 5.1.0 to reflect new linear interpolation functions
Fix dimension checks for interp.lin2d and interp.lin3d
Document linear interpolation functions
Fix name for interp.lin3d
Add 2D and 3D linear interpolation
Initial values are the mean of nrep prior draws, which diverges when the prior has no finite mean