get a cell, return default if that cell does not exist
                        note that column and row START AT 1 same as excel

def GetCellValue (self, column, default = None):
                """ get a cell, return default if that cell does not exist
                        note that column and row START AT 1 same as excel
                """
                if isinstance(column, str):
                        column = ColumnToIndex(column)
                if column in self:
                        if "content" in UserDict.__getitem__(self, column):
                                return UserDict.__getitem__(self, column)["content"]
                return default