[Proj] How grids work in PROJ4

[Michal Seidl]

Hi everybody,
finally I hope it is clear to me.
- the +geoidgrids is implemented correctly and works as Frank wrote
- Mikael Rittri explanation helps me a lot, I was really for a moment 
confused by Frank's one
- Joaquim formula h = H-N is OK. But as Frank said +geoidgrids is only 
name and can be used for other height correction which is exactly my 
situation.

Thanks a lot Michal