Convert `dt.isna()` to `FExpr`

[samukweku]

WIP for #2562

[oleksiyskononenko]

The reason this build is failing may have nothing to do with this PR

[2023-03-15T18:45:15.448Z] Cancelling nested steps due to timeout

[Pipeline] // node

[Pipeline] }

[2023-03-15T18:45:15.469Z] Failed in branch Build on x86_64-macos

When you push more updates, the build will be re-triggered and we will see if this was just a temp glitch on Jenkins.

[oleksiyskononenko]

@samukweku As a part of this PR, let's not deprecate dt.isna() because it seems that it was deprecated by mistake. For the moment, let's keep both dt.isna() and dt.math.isna(). Hence, the corresponding f-method could be simply called .isna().

[oleksiyskononenko]

Thanks for the changes, is this PR now ready for a review?

[samukweku]

@oleksiyskononenko yet it is ready for a review

[oleksiyskononenko]

So I propose we keep dt.isna() as a main function, and for others (dt.math.isna() and f.isna()) say they are "the same as dt.isna()".

[oleksiyskononenko]

Not sure how you merge main, but it seems that you're just adding more commits on top of this branch. What you need to do is

git checkout main
git pull
git checkout samukweku/fexpr_isna
git merge main

This will create only one merge commit incorporating all the changes from main.

[samukweku]

i'm using the code from the guide:

git checkout main
git fetch upstream
git merge upstream/main
git checkout samukweku/fexpr_isna
git merge main

[oleksiyskononenko]

I guess the guide explains how to start a new branch and not how to merge existing changes onto the existing branch.

[samukweku]

@oleksiyskononenko I have made the changes. Do let me know if there are other parts that need fixing. thanks

[samukweku]

@oleksiyskononenko made edits to cover for non FExprs for isna. kindly have a look when you can.

[samukweku]

@oleksiyskononenko just checking in on your feedback