deprecate stackdf and meltdf

[bkamins]

Fixes #1736. It does not have to go into v0.20 if some problems are found.

[nalimilan]

Good idea!

[bkamins]

OK - all should be fixed. I just used _ in front of internal functions _stackview and _meltview.

[bkamins]

Only coverage fails here.

[bkamins]

I will add deprecation of melt and meltdf in this PR as it duplicates stack and stackdf and we do not need both since we have Not for indexing. I will let you know when this is implemented.

[nalimilan]

Maybe merge this now and file another PR as this one seems to be ready.

[bkamins]

Thank you for having a look at this.

I had to clean up this PR anyway (I have left some references to stackdf in the docs - sorry for this) so I also deprecated melt. This is a rather simple fix, as it is either adding Not to an argument or changing argument order (you can see the changes in deprecated.jl).

[bkamins]

This should be good to have a look at. I also wanted to have it in one shot as previously we changed methods that are removed entirely now.

[nalimilan]

Looks good with some doc fixes.

[bkamins]

Incorporated all changes. Thank you for reviewing this.

[bkamins]

Thank you!