Update tutorials for v0.4.2

[nkeim]

Lots of changes—a refresh was long overdue!!

Walkthrough

• Use pims.open() and makes a pipeline instead of as_gray=. Fixes https://github.com/soft-matter/trackpy/issues/505 , fixes https://github.com/soft-matter/trackpy/issues/532
• Mention possibility of gray values in range [0, 1]. Resolves https://github.com/soft-matter/trackpy/issues/537 , resolves https://github.com/soft-matter/trackpy/issues/541 , resolves https://github.com/soft-matter/trackpy/issues/543
• Replaced link_df() with link()
• Removed deep links into v0.3.0 docs
• Added link to Savin & Doyle
• Added tip about threshold argument - resolves https://github.com/soft-matter/trackpy/issues/157
• Updated minmass and size cutoff values
• Corrected several typos (thanks to @mmoelter)
• Added a little more explanatory text in several places
• Added link to Catipovic, M. A., Tyler, P. M., Trapani, J. G. & Carter, A. R. Improving the quantification of Brownian motion. American Journal of Physics 81, 485-491 (2013).
• Added advertisement for new parallel batch.

Note that the power law results have changed, from n = 1.04738, A = 1.665085, to n = 0.962912, A = 1.969866 . (Nominal values n = 1, A ~ 1.66.) I haven't found an easy way to get them closer to previous values. I've tried casting images as uint8 (as in the old version) and messing with the cutoffs—any ideas?

Other highlights

• Updated pims usage in a few other notebooks
• In custom features tutorial, added faster velocity field calculation by @Thefalas to resolve https://github.com/soft-matter/trackpy/issues/524

[caspervdw]

@nkeim I started the notebooks locally and read through them. Thanks for this effort of making them up to date. I didn't have any comment, so merging.

[nkeim]

Thanks @caspervdw !