Updated "generalize" methods

See the comments for what has been changed.

Try:

```
library(spacetime)
example(Track)
g1 = generalize(B, max, timeInterval = "2 min")
g2 = generalize(A2, distance = 200)
g3 = generalize(Tr, min, n = 2)
g4 = generalize(A, timeInterval = "3 min", tol = 2)
g5 = generalize(A1, n = 3, toPoints = TRUE)
```