Feature request: pull and push with %%px cell magic

I propose allowing one to pull and push in a px cell. The feature allows better encapsulation of cells.

example:
```
%%px push(a, b) pull(c)
c = a + b
```
translates to something like
```
client.push(['a', 'b'], block=False)
%px c = a + b
client.pull('c', block=False)
```

Start by locating the %%px cell magic implementation and how it currently parses and executes a cell. Compare its behavior with the proposed push(a, b) and pull(c) syntax in the issue example; done means a cell can transfer the named variables before and after remote execution as shown.

索引モデルが issue の本文から書いたものです。