🎉 Functional flow execution

[AlexandreSajus]

A code block now has two buttons:

• Left run: will run the block and all of its dependencies in the right order
• Right run: will run the block's outputs and all of their dependencies in the right order

This is done through an execution queue and graph traversal utilities

Closes #48
Closes #25 executing two blocks at the same time will now create a queue

Still lacks:

• testing (testing can be done manually using the two added examples and the modified mnist)
• not running a cell when it has been run and was not modified

[MathisFederico]

We need to add block.has_ran property and refactor