splash to cylindrical grid seems to cut off at -ve z boundary

[danieljprice]

Issue raised by Geoff Bryan:

something is wrong with 3D interpolation to cylindrical grid, particles do not seem to interpolate correctly near -ve z boundary. Problem always occurs near -ve z boundary no matter where it is placed. Can be seen in attached png file which shows single slice (constant phi) from 3D r-phi-z interpolation (64 x 64 x 64)