Transpose tests and table column bugfix

[OssianEPPlus]

ShowHeaders = True and saving no longer causes crash if table has same column names. It also no longer overwrites cell values in empty cells when set to true. That behaviour now happens on OverwriteColumnNamesWithCellValues or on OverWriteCellsWithColumnNames if values already exists in the cells.

Loading in names now works as such:

1. Loading in column names that are duplicates of other names results in the column name ColumnX where x is the column number. Alternative ColumnX2 if it already exists.
2. Applying a new column name that already exists in the table throws a more readable error
3. Adding a column gets name ColumnX or ColumX2 if it already exists.