Spreadsheet functionality #2

[portnov]

See previous iteration on this in #3639.

I discovered that there is a python library, called pyexcel, which allows us to read and write XLS, XLSX, ODS and CSV files.
So now, in Sverchok-Extra there is a new node, "Read Excel". It can output data as nested lists, as dictionary by rows or by columns, or as a nested dictionary. It can be used together with Spreadhseet node.

Excel data:

Example of usage of that data, with calculations in usual Sverchok nodes:

Similar setup, but now all calculations are done in Spreadsheet node (this is the simplest example of it's usage):

Now there is also a "Write Excel" node as well.

The question remains: do we need all these nodes (Data Item, Spreadsheet, Read Excel, Write Excel) in main Sverchok? Or let them remain in Sverchok-Extra?