Strict types

[martinRenou]

I would like ipysheet to go in the direction of static typing for cells. e.g. if a cell is of type numeric, it would not be possible to set a string value, whether the user set it from Python or from the GUI.

• We can disallow invalid cell values setting the allowInvalid parameter to false in Handsontable. This prevents the user from typing a string in a numeric cell.
• We can use Traitlets validators in order to check that the cell value type makes sense according to the cell type.

Also, I would like to remove the numeric cells support, while adding two new cell types in ipysheet: int and float, with custom validators and editors.