ipympl when jupyter and ipython kernel are in different conda environment

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Hi,

## Some context

I typically install the jupter notebook in a conda environment, and create a shortcut on the desktop to launch jupyter-notebook.exe (windows 10 64 bits). Then I create an ipython kernel with all dependencies I want to work with in an other environment, and I register this kernel-in-a-conda-environment with the command

```
ipython kernel install --user --name NameOfMyNewKernel
```

I find this workflow relatively clean because I can chose when I want to update either my kernel (bleeding edge or very old depending on this situation) independently from the notebook itself.

## The problem

As far as I understand `ipympl` is really 2 packages.

- One is a python package called ```ipympl``` that must be installed together with your `ipython` kernel, because you need to write `import ipympl` in your notebook.
- The other one is a javascript package (I supposed it is called also ipympl) that must be installed together with the jupyter notebook application. I don't know if the python package is required or not. You also need to make sure that the javascript package is registered as an _extension_ (`jupyter nbextension enable --py --sys-prefix ipympl`) by jupyter.

If this is documented somewhere it is well hidden ;-)

## The workaround

I installed 2 times the ipympl package, one in my kernel environment, the other in the jpuyter environment. For the latter case, because the python package is also shipped in the ipympl conda package, matplotlib is installed but I think that is useless. With this, everyting works, I am able to see interactive plots in my notebook.

Before figuring out that I needed to install `ipympl` in the jupyter environment, I received weird error messages without really explaining what the problem was.

## Proposal

1. Document the problem. I suppose that this repository README coud explain this, but I suppose it should be also documented somewhere in matplotlib and/or jupyter documentation. I must admit I find the jupyter documentation really hard to navigate though.
2. Split ipympl conda package in two: one that contains the python package and the other one containing the javascript package.

Thanks

Inizia esaminando il README del repository e i comandi documentati di conda, Jupyter kernel e nbextension menzionati nell’issue. Chiarisci quali parti di ipympl appartengono all’ambiente del kernel e quali all’ambiente Jupyter, e documenta la configurazione necessaria e i sintomi dei malfunzionamenti. Valuta inoltre se sia fattibile suddividere il pacchetto conda; il lavoro è completato quando il workflow con ambienti separati è spiegato chiaramente.

Scritto dal modello di indicizzazione a partire dal testo della issue.