Fix arrays writing with PyTelTools and other improvements

[lucduron]

I am not able to run tests and dataset.equals does not seem to work on my environment (it raises a numpy.exceptions.AxisError).

But what has been tested so far:

• test_open_dataset: OK
• test_to_selafin: OK (same binary input/output file)
• test_to_slice: OK only for time selection

[tomsail]

It works great thanks @lucduron (I also tested on different files).

[tomsail]

There is still work to do on the dask/lazy indexing part though..
But I'll merge your changes already