Install ipython dagster kernel if not present

[mgasner]

Resolves #908

[coveralls]

Pull Request Test Coverage Report for Build 105762

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.092%

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Totals
Change from base Build 105070:	0.0%
Covered Lines:	7797
Relevant Lines:	9163

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💛 - Coveralls

[schrockn]

we should document what is going on here. this is only for the development environment correct? we don't assume the "dagster" kernel name in the dagstermill core

[mgasner]

Dagstermill tests invoke notebooks that look for an ipython kernel called dagster -- if this is not already present, then the tests fail with a cryptic error (#839). In the status quo, this will generally happen on first download / install -- with this diff, that kernel is created if it is not already present before tests run.

[mgasner]

ping @schrockn

[schrockn]

So we are committing to the fact that the user must have a hardcoded kernel named dagster in the system?

[mgasner]

I could look for a kernel named dagster, and if we don't find one, then overwrite the kernel in the test notebooks with a kernel that they do have (?) -- then rewrite it at test cleanup.

[mgasner]

nteract/papermill#338

[schrockn]

Where would be a good spot to communicate to the user what we are doing. Somewhere that prints out "We have automatically created a kernel named "dagster". <Insert some explanation of what a kernel is. If you have your own kernels you want to use, please edit the notebook to do it"

Or something of the sort.

[mgasner]

I've added a warning... the nice thing about warnings is they do print by default in pytest.

[schrockn]

👍🏻