'datasets.py', preview working, still some bugs
start replacing field 'ContentsFormat' with 2 fields 'Format_Python' and 'Format_Excel'
'datasets.py': Enable excel formats in addition to text, defaults for new column
'datasets.py', streamlined Sheets to use dictionary, removed redundant code
'datasets.py', fixed bugs: min/max numeric not alphabetical, error on blank columns
applied formats; fixed bug for empty sheets