Possibility to add a custom block in the GUI

[RemDelaporteMathurin]

Right now, to add a new block implementation (eg. a subclass of Function) users need to add it to the source code of pathview with extract-blocks.py.

It would be extremely valuable to be able to add a new block "out of the box" directly in the GUI.

We could imagine a "Create your own block" wizard where users would:

• provide the python definition of the block, for example:

from pathsim import *

class NewBlock(Function):
    def __init__(self, ...):
        ...

• provide some UI overrides, for example: maximum number of inputs is 3
• select a shape (or even upload their own svg!)

Then users would be able to use this block in their model but also:

• save it to a file
• import it later