Enable usage in google colab

[PythonFZ]

Google Colab does not support nbconvert so we can not convert the notebook to python files. It would be very nice to allow ZnTrack usage in Colab so here are some possiblities:

• Write Python files in the beginning of the notebook
• define e.g. __src__ which contains the Node src including the required inputs

• Use something like this

  @NBNode   # where to put the decorator / decorators ?
  def func():
  import zntrack
  @Node()
  class HelloWorld:
  def print(self):
    print("Hello World")
  
  return HelloWorld
  print(inspect.getsource(func))

  and potentially fix #103

[PythonFZ]

see https://stackoverflow.com/questions/51566497/getting-the-source-of-an-object-defined-in-a-jupyter-notebook for a better solution