Fix parsing dimensions in xlsx files

Note that this pull request is missing tests, but I want first to check that my approach is fine. The problem is that some xlsx files have the following start:

```
<?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">
<sheetPr filterMode="false">
<pageSetUpPr fitToPage="false"/>
</sheetPr>

<dimension ref="1:49"/>

<sheetViews>
<sheetView colorId="64" defaultGridColor="true" rightToLeft="false" showFormulas="false" showGridLines="true" showOutlineSymbols="true" showRowColHeaders="true" showZeros="true">
<selection activeCell="A29" activeCellId="0" pane="topLeft" sqref="A29"/>
</sheetView>
</sheetViews>
<cols>
<col collapsed="false" hidden="false" max="1" min="1" style="0" width="51.9333333333333"/>
<col collapsed="false" hidden="false" max="2" min="2" style="0" width="21.8313725490196"/>
<col collapsed="false" hidden="false" max="3" min="3" style="0" width="22.5647058823529"/>
<col collapsed="false" hidden="false" max="1025" min="4" style="0" width="11.6745098039216"/>
</cols>
```

note that dimension is "1:49", while xlrd expects it to be of the form "A1:D49".

In my patch I add an argument for cell_name_to_rowx_colx, which allows parsing cell names with missing columns, and if we have no information about column range in this file, we just do not set them.

If this is fine, I will add direct tests for cell_name_to_rowx_colx (currently, there are no such direct tests).