Add support of sparse sheets

[charphi]

Sparse sheets describe 2 cases:

1. top-left cell is not A1
2. rows have variable number of cells (see spans)

Example:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="x14ac xr xr2 xr3" xmlns:x14ac="http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac" xmlns:xr="http://schemas.microsoft.com/office/spreadsheetml/2014/revision" xmlns:xr2="http://schemas.microsoft.com/office/spreadsheetml/2015/revision2" xmlns:xr3="http://schemas.microsoft.com/office/spreadsheetml/2016/revision3" xr:uid="{33072E22-ADF7-419D-8ECE-38FFE27B1E26}">
	<dimension ref="C3:F6"/>
	<sheetViews>
		<sheetView tabSelected="1" workbookViewId="0">
			<selection activeCell="F6" sqref="F6"/>
		</sheetView>
	</sheetViews>
	<sheetFormatPr defaultRowHeight="15" x14ac:dyDescent="0.25"/>
	<sheetData>
		<row r="3" spans="3:6" x14ac:dyDescent="0.25">
			<c r="C3" t="s">
				<v>0</v>
			</c>
		</row>
		<row r="6" spans="3:6" x14ac:dyDescent="0.25">
			<c r="F6" t="s">
				<v>1</v>
			</c>
		</row>
	</sheetData>
	<pageMargins left="0.7" right="0.7" top="0.75" bottom="0.75" header="0.3" footer="0.3"/>
</worksheet>