Better handle unicode column names

[maartenbreddels]

Creating a dataframe with unicode chars like this:

df = vaex.from_dict({'ÚÚ': vaex.vrange(0, 100)})

Will replace the two characters with two underscores, which effectively hides the column (column names beginning with __ are hidden).
This was mentioned here:
https://stackoverflow.com/questions/59738879/python-vaex-how-to-create-dataframe-from-a-csv-file

[markkoob]

I think this will be obvious (to you!) but since I am here anyway:

df = vaex.from_dict({'./label': vaex.vrange(0,100)})

In our case these characters have special meaning for us, so mutating the string is pretty inconvenient. I'm hoping the solution involves preserving the original string!

[maartenbreddels]

Thanks for sharing that.

To give some understanding, vaex uses the column names as variable names, thus they have to be valid identifiers (like variable names), since it's all built on valid Python expressions. In #370 we improved this to provide automatic translations, but it seems we need to do some more work.