`extrapolate` not used in tracking

[freemansw1]

I think that this gets back to the discussion in #149, but the extrapolate parameter is not currently used in linking_trackpy. We should probably warn users when using it at the moment until #149 is resolved. I'm going to ping Max to see if he has thoughts on #149.

[snilsn]

Agreed, and I think the same applies to the order parameter.

[freemansw1]

After some discussion with users, I think we should take the following actions on this:

• [x] Raise a NotImplementedError when users try to set extrapolate in v1.4
• [ ] Implement extrapolation (rather interpolation, really) and fix the fill_gaps function at some point in the future

Please, let me know thoughts!

[freemansw1]

#177 will fix the first part of this for v1.4. I've moved this now to v1.6 for the second part of the issue.