Implement new alternate class table writing method

[DavidStirling]

Should fix #314

It turns out the old ('backup') method for writing class tables just doesn't work with the chunking functionality added in a recent version. To be honest the approach used by that method is overly complicated and much slower than it should have been. I've therefore just written a new non-Pandas method of putting the class data into the class table.

It's worth confirming whether this resolves the issues previously seen by users. It's not obvious what error would cause the Pandas-based method to fail, so it'd be worthwhile to investigate what the actual exception was that caused it to fall back to the alternative writing method in the first place.

I've also moved a bunch of print statements into the debug logging level so that the user can see them if desired.