link_df() with diagnostics=True: works with pip trackpy, breaks with github trackpy

[charlesreid1]

I noticed several example notebooks (particularly, linking-diagnostics) using a legacy argument to link_df(), namely diagnostics = True. These notebooks import the regular link_df(), but the diagnostics = True keyword has recently been removed. This is not a problem when using the pip version of trackpy (where the diagnostics keyword is still there), but it breaks the examples when using the latest github version of trackpy.

I plan to fix this with existing pull request #44 but I wanted to suggest a "recommended" method for users to import the legacy version of link_df(), in case they do want diagnostics. Here's what I did:

import trackpy as tp
import trackpy.linking.legacy as tplegacy

...

t = tplegacy.link_df(f, 13, memory=3, diagnostics=True)