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: ```python @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 ```
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