remove contour plot methods

[tdhock]

Maybe remove contour plot support/examples?
https://cloud.r-project.org/web/packages/isoband/vignettes/isoband3.html
does much better labels.

[ANAMASGARD]

Sir @tdhock
I opened PR #62 for this. It deprecates top.pieces/bottom.pieces (adds .Deprecated() warnings), removes the contourplot section/examples from the docs, and adds a README note pointing users to isoband’s labeled isolines.
Could you please review when you have time and aslo please run the workflows ?