This function returns the number of spatial coordinate dimensions
  of the dataset X. The results for some of the more common
  types of objects are as follows:
| object class | dimension | 
| "ppp" | 2 | 
| "lpp" | 2 | 
| "pp3" | 3 | 
| "ppx" | number of spatial dimensions | 
| "owin" | 2 | 
| "psp" | 2 | 
Note that time dimensions are not counted.
If X is not a recognised spatial object, the result is NA.