Update SimpleXLSXGen.php

[xaviermdq]

Minor changes:

1. Method with no interaction with object properties/methods, converted to static method: num2name (used in _sheetToXML).
2. Constructor: It's not necessary to initialize the $sheets property since, in the addSheet method, the index 0 of $sheets is overwritten ($curSheet initialized at -1 and becomes 0 when addSheet is called).
3. Minor XML formatting changed (newlines and light indentation): in $template definition (constructor), $SHEETVIEWS, $ROWS, $COLS, $AUTOFILTER, $HYPERLINKS (_sheetToXML), etc.
4. addSheet method: $rows initialization changed to support associative arrays. In the original addSheet method, it is checked if $row is a 2-dimensional array but considering $row as a classic array (2-dimensional array with consecutive numeric indices). But traversing data from the array (in the _sheetToXML method) is done with foreach, which allows the use of arbitrary indexes. So, I think a more correct way to check $row presence is to use:

if (is_array($rows) && is_array($rows[array_key_first($rows)]))

but it needs PHP 7.3 so we can use:

if (is_array($rows)) {
	foreach ($rows as $row) {
		if (is_array($row)) $this->sheets[$this->curSheet]['rows'] = $rows;
		break;
	}
}

5. saveAs and downloadAs methods: change filename correction to a more aggressive aproach so can be saved in *nix/Win filesystems. Both methods return false on failure or used filename on success.
6. _write method: search & replace sentences changed to one multi-line sentence (I think it gives better readability).
7. Changed coord2cell/cell2coord methods to use array for sheet coordinates. This caused a change in the _sheetToXML method where these methods are used to calculate frozen cells.
8. Method _sheetToXML: added empty element at the end of the arrays $ROWS, $COLS, $MERGECELLS, $HYPERLINKS (like it was originally with $MERGECELLS) and deleted the new lines in the sheet template.
9. The documented methods (and class), converted to PHPDoc format.
10. date2excel method: $century vriable used to calculate $decade to avoid repeating calculation.

I have removed the code related to comments in cells to make it easier to see the changes. In a later version, I will add comment support.
Also, and for the same reason, I did not change the order of the methods but it would be nice to group the static methods, the setters and the rest.