Support empty column names & duplicate column names

[tebeka]

We need to decide what to do with empty column names.

Looking at pandas:

• read_csv will automatically assign names to columns if it's empty.
• If there's a duplicate column names, and you try to get it - pandas will return a column (probably the first) that matches this name

[tebeka]

IMO the simple solution is to disallow column names and duplicate names. This will make things much simpler for us. This will move the logic of what to do with empty column names in the clients and the backends.

• In Python we can check at the client level that the DataFrame comply
• In Go NewFrame and friends can check it.

@yaronha - Thoughts?