[cleanup] Remove some out-commented debugging code
[cleanup] char* --> const char*
[cleanup] Make method PriorityMerge return enum PrioMergeVals instead of plain 'int'

Because the return values really are such enumeration values.
Constify one argument of method XferUnpack

For better readability of the code
Remove some out-commented debugging code
More constification
Add two explanatory comments
Assert that overwriting a certain object ddd header does not change its ddd type

This makes my current 'rising droplet' adaptive load rebalancing test case
crash earlier.
[cleanup] Use BOOL instead of INT for the with_vector argument
Use enum IdentMode more often, instead of plain 'int'
[cleanup] Remove useless preprocessor conditional
Make a pointer method argument const
[bugfix] Initialize EDIDENT for edges for 3d _and 2d_ before refinement

Previously this was only done in 3d, because edges where second-class citizens
for ddd (they didn't have a DDD_HDR).
[bugfix] Alway identify MIDNODEs to the proclist of the father edge

Previously this was only done for 3d grids.  However, edges in 2d grids
are now also ddd objects (the have a DDD_HDR), hence they can be used
for identification.
[cleanup] Remove some debugging output
Fortran compatibility (PPIF_FRONTEND_F) removed