Make <OptimizationItem>.data a pd.DataFrame in the core layer

[glatterf42]

In his review of #101, @danielhuppmann noted that we should return a pd.DataFrame for e.g. optimization.indexset.data, at least in the core layer. This would retain consistency with ixmp and we expect most users to want a pd.DataFrame. Should they require a dict, the conversion can still be done in one line, much as it is now.