[VEX]primvertexcount, Get number of vertices in a primitive in geo.

------- primitive
int  primvertexcount(<geometry>geometry, int prim_num)

ex1).
int        nvtx;
// Get the number of vertices of primitive 3
nvtx = primvertexcount("defgeo.bgeo", 3);
------- 
ex2).
if(primvertexcount(0, @primnum)==4){
    removeprim(0, @primnum, 1);
};