Add info query for Excel files

[davidmegginson]

Add an exportable function that will scan an Excel file and return a list of dicts (one for each sheet) with the following information:

• sheet_name (string)
• has_merged_cells (boolean)
• is_hxlated (boolean)
• header_hash (MD5 string)
• hashtag_hash (MD5 string, or null if not HXLated)

To support https://github.com/HXLStandard/hxl-proxy/issues/377

[davidmegginson]

In the end, added an info() method to hxl.input.ExcelInput that returns a list of dicts, one for each sheet, with the following properties:

• name (string): the sheet name
• is_hidden (boolean): True if the sheet is hidden
• nrows (int): number of rows
• ncols (int): number of columns
• has_merged_cells (boolean): true if there are any merged cells in the sheet
• is_hxlated (boolean): true if the sheet can be parsed as HXL data
• header_hash (MD5 string): a hash for the first row of the raw datasets
• hashtag_hash (MD5 string): a hash for the HXL hashtags/attributes and attached text headers, or None if not HXLated

It will be simple to add more information in the future, such as frozen rows and columns. This should support everything needed in https://github.com/HXLStandard/hxl-proxy/issues/377

[davidmegginson]

Modification: the list of sheets is under a property "sheets". A top-level property "format" also appears (currently "XLSX" or "XLS", but could be other formats in the future). That also leaves room to add additional top-level properties.