WIP: add python eval/exec to cells

[chaffra]

Hello,
I'd like to propose this PR. It's still a WIP but the idea it to be able to type in python expression in cells and have them executed by the Cell widget on the backend. The backend part seems to be working. You can do something like:

sheet1 = sheet(rows=2,columns=2)
display(sheet1)
c00 = cell(0,0,value=1.0)
c01 = cell(0,1,formula='S[0,0].value*2')

If you then change c00, c01 is updated.

I'd like some guidance working on the js side. How do I modify the hot object so that when I edit a cell it shows the attribute formula instead of value?

[shaunc]

Great idea! :)