def cell(row, col, sheet = nil)
      key = normalize(row, col)
      safe_send(sheet_for(sheet).cells[key], :value)
    end
def cell(row, col, sheet = nil)
      key = normalize(row, col)
      safe_send(sheet_for(sheet).cells[key], :value)
    end
Returns the content of a spreadsheet-cell.
 (1,1) is the upper left corner.
 (1,1), (1,'A'), ('A',1), ('a',1) all refers to the
 cell at the first line and first row.