Python and node call, follow PyCall's example

[tlienart]

To avoi issues with people not having python3 or node as a way to call the relevant executable, it would be better to follow the route explained in https://github.com/JuliaPy/PyCall.jl where one can specify ENV and change whatever path they want.

See also comment in #159

In windows' powershell, it looks like py -3 is the right way to call things (see https://stackoverflow.com/questions/20364207/how-do-i-add-python-3-3-to-powershell) so there may need to be a system test etc. Maybe specifying ENV is the easier route.