[OperatorImpl] Python OperatorImpl

• Bind OperatorImpl so that it can be extended on Python side
• Add Operator.setImpl method
• Replace std::getenv("AIDGE_CORE_WITH_PYBIND") with Py_IsInitialized() (fix #24 (closed))
• Add unittest

Usage example

import aidge_core

class PythonReluImpl(aidge_core.OperatorImpl):
    def __init__(self, op):
        aidge_core.OperatorImpl.__init__(self) # Recquired to avoid type error !
    def forward(self):
        print("Forward defined in Python !")

custom_relu = aidge_core.GenericOperator("Relu", 1, 1, 1, name="myReLu")

custom_relu_op = custom_relu.get_operator()

custom_relu_op.forward()
custom_relu_op.set_impl(PythonReluImpl(custom_relu_op))
custom_relu_op.forward()