Gh issue 681 pt2

[JanMarvin]

In addition to #681.

• remove deprecated function read_sheet_names() (it looks horrible and was deprecated in 0.7?)
• make wb_get_sheet_name() internal. There is a wrapper function wb_get_sheet_names() which is just an s away.
• change xlsx_file to file, another option I was considering is wb. Because file can be a file or a workbook and wb kinda reflects that. Thoughts?

[sjewo]

My code broke because of the removed read_sheet_names() function and it took some time to find out what the problem was.

So I leave this here for further reference:

# replacement for read_sheet_names
wb_get_sheet_names(wb_load(filepath))

[JanMarvin]

Thanks!