Mostieri/shared memory interface

[mariostieriansys]

This PR enables the shared memory interface via PyEnSight.

There are two services available:

1. The user can "subscribe" to an image stream. In this case, the user can then choose if to use a shared memory client or not. In the latter, the code tries to launch the client on the same Python process the PyEnSight session is running. In the worst case, it will launch it on the remote EnSight. This is not mandatory, since the image can be streamed via the gRPC layer if use_shmem is False
2. The user can enable directly an image stream. This doesn't launch any shared memory client, but the stream is enabled via a different gRPC request, and the images served via the gRPC layer.