Add method to check if a cell is merged
Ignore merged cells when calculating column width

When calculating column width cells that are merged should be ignored. This is MS Excel behavior and should be handled similar in ABAP2XLSX.
Add method to check if a cell is within a range
Add test method for is_cell_in_range
Add sheet for merged cells

I have added a new sheet to demonstrate that merged cells are ignored when using auto-size (MS Excel like behaviour).
Merge pull request #333 from oliver-huetkoeper/master

Ignore merged cells when calculating column width