nicyfication of mechanism for copying inst. and stat. variables from local to compound block structure

- using index mapping via the explicitly computed variables ia1:ie1 -> ia2:ie2 , ja1:je1 -> ja2:je2 , ka1:ke1 -> ka2:ke2
- also some cosmetics for the code and safety checks
revamped logic for computing indices in local / compound block structur

- introducing left / right , bottom / top , front / back variables to indicate if a block sits on the boundary of the compound domain
- buffer cells are automatically included for boundary blocks
- no need to explicitely call FILLBUFFER_* anymore,
  thus the grid files can be left untouched when postprocessing only parts of the complete domain