DataFrame.delevel improvements

[wesm]

from @lodagro

Concerning the method name, DataFrame.delevel() is maybe too close to a MultiIndex. 
DataFrame.deindex() is more general, both are fine for me. And i have no problem to change either.

One little remark, for a MultiIndex default column names are inserted when needed, whereas if no MultiIndex is used, and no name - an exception is raised. Would it not be cleaner to do the same for both, either raise an exception if no names are set or enter default column names?

[wesm]

I took a hack at this. I don't think the default name matters all that much unless it's consistent-- it will be 'index' unless that's taken (relatively unlikely, but...) in which case it will be 'level_0'. I'll go with that unless you have strong feelings to the contrary

[lodagro]

I agree, default naming not so important, consistency counts.