Let's figure it out ...

I've added the possibility to allow for selections as shown below:

```
Tr[list(1:2, 2:3)] # Select the first two tracks from ID1, and tracks 2 and 3 from ID2.
```
I'm sure the code could be more efficient (I've tried to replace the for loop, but didn't work out unfortunately, mainly due to the mandatory (?) usage of the index), but at least it's working.

I've also added coercion methods that allow for converting tracks to SpatialLines objects. See the source code for comments and open issues!

I thought providing an over method for TracksCollection objects would be pretty straightforward leveraging the results from above, but as far as I can see it there's no over method for SpatialLines objects?
```
showMethods("over")
```