More efficient Worksheet methods: min_row, max_row, min_column, max_column

Existing implementation is building a set of all distinct row/column numbers, which is not necessary. We can make it much faster like this:

    @property
    def min_row(self):
        """The minimium row index containing data (1-based)

        :type: int
        """
        min_row = 1
        if self._cells:
            min_row = min(c[0] for c in self._cells)
        return min_row


    @property
    def max_row(self):
        """The maximum row index containing data (1-based)

        :type: int
        """
        max_row = 1
        if self._cells:
            max_row = max(c[0] for c in self._cells)
        return max_row


    @property
    def min_column(self):
        """The minimum column index containing data (1-based)

        :type: int
        """
        min_col = 1
        if self._cells:
            min_col = min(c[1] for c in self._cells)
        return min_col


    @property
    def max_column(self):
        """The maximum column index containing data (1-based)

        :type: int
        """
        max_col = 1
        if self._cells:
            max_col = max(c[1] for c in self._cells)
        return max_col