Raise exception on non-unique column index in to_hdf for fixed format.
Fixes #7761.

hmm, maybe make this a `NotImplementedError` (as its not invalid, just not supported). Also maybe a note in the io.rst (in the Fixed section)
 Well, with the current fixed format it is in fact invalid. There is no way to get this right without changing `block_items` to point on columns using integers, i.e. their position in the zeroth axis.
 it _could_ be implemented (it just isn't). You are preventing the writing (which invalidates the reading) either way. The exception will be more informative (`ValueError` is saying something about the input is not valid. It IS valid, just not in writing :).

Actually you might want to have a nice exception message saying that you CAN store this in 'table' format.
 It could not be implemented because `block_items` contains only strings. You'd have to change the fixed format to implement this, for all other cases (i.e. append and compression) a `ValueError` is raised.

If this is the only thing keeping you from merging I'll change it to `NotImplementedError` ;)
 @filmor thanks for this!
 this is technically for a non-unique info_axis (e.g. columns for df, but 0 axis for Series/Panel). Can you add some tests for this? And update the note in v0.15.0 (I moved it a bit)
