Merged Cells in Excel

### Question
1. When using the following code to parse an Excel file:
converter = DocumentConverter(
allowed_formats=[InputFormat.XLSX],
format_options={InputFormat.XLSX: ExcelFormatOption(pipeline_cls=SimplePipeline)},
)
If the sheet contains merged cells in the header, the parser recognizes it as multiple tables. Is there any way to make it recognize only one Markdown table per sheet?
2. By the way, is there a parameter to specify a particular sheet to parse? Not for all Excel sheets.
3. The output JSON from the `export_to_dict` function is a bit too complex, could provide a simple explanation of what each variable represents? My goal is to extract tables from Excel in a row-based JSON format, but currently, I can only achieve this by exporting to Markdown and then splitting the content by blank lines :(