Support slices to be used in xsuite

Slice should be an element that tracks a portion of a field expansion. Something like this should work

```
Slice:
     def __init__(self,hamiltonian,s_start, length):
        ....
     def track(...):
          s_span = (s_start, s_start+length)
          self.hamiltonian.track(..., s_span=s_span)
```