Fix DataFrames v0.19 deprecations

[alyst]

Switches from haskey(df, colname) to hasproperty(df, colname) and from df[colname] to df[!, colname].
I guess that requires DataFrames v0.19 (or a bit earlier), but since it's an optional dependency, the check has yet to be coded in __init__(), which I can try to implement if needed.

[sglyon]

Hey thanks!

This is great.

Would you please note in the project.toml file that these changes require DataFrames version >= 0.19?

[alyst]

I've updated Project.toml, but, as I mentioned before, I'm not sure it's gonna work (in the ]test PlotlyBase I don't see that DataFrames.jl is used/or its version checked).
See also JuliaPackaging/Requires.jl#3.

[sglyon]

Thanks!

[sglyon]

oh shoot, you are right . Let's go back to the first commit.

My mistake! Sorry about that

[alyst]

No problem. Actually, maybe it's nice to keep these changes to Project.toml just as a future note?
So far they look dormant and should not negatively affect versions resolution.
Unfortunately, we cannot add clarifying comments to Project.toml as Pkg will strip them.