fix async for usage with python 3.7

[douglasrizzo]

In Python 3.7, async is a reserved word, so I changed it (there were only a few occurrences) to asynch. This has only affected .py files.

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[douglasrizzo]

I just tested it (without GPU) on Python 3.7 and everything works fine. I believe this fixes #124 and #127.

[douglasrizzo]

OK, my fix didn't work when using the GPU.

I took a look at the documentation for the pytorch.Tensor object and, apparently, the async argument is not even used in the copy_ and cuda methods (which are the ones used in ELF), so I just deleted them. Now everything works.

[douglasrizzo]

I had a problem on my side by pushing from my master branch. I created a new PR #135 from a new branch now