Row.PushCell leads to Memory Leak

[liangdy678]

Hi，
when we merge some xlsx files or copy some cells，memory leak occurs in some application scenarios.

// PushCell adds a predefiend cell to the end of the Row
func (r *Row) PushCell(c *Cell) {
	r.cellStoreRow.Updatable()
	r.isCustom = true  
	r.cellStoreRow.PushCell(c) 
        // c.Row Still only points to the original Row.
        // c.Row -> c.Row.Sheet -> c.Row.Sheet.File , then blocks GC
}

we recommand adding a new function CopyCell ：

// CopyCell adds a predefiend cell to the end of the Row
func (r *Row) CopyCell(c *Cell) {
	r.cellStoreRow.Updatable()
	r.isCustom = true
        c.Row = r   // Release the reference to the original row，points to the target Row
	r.cellStoreRow.PushCell(c)
}

[tealeg]

@liangdy678 - yes, I see. I hadn't anticipated this use case. I'll consider this for either v4 or the next minor release.