DISC: Update `jupyter_kernels` option

[mmcky]

@AakashGfude let's review jupyter_kernels option. We currently specify a few settings for setting up a ipynb metadata.

• [x] Is this option still used when constructing the notebooks?
• [ ] can we infer jupyter kernel details from jupyter?

# Configure Jupyter Kernels
jupyter_kernels = {
    "python3": {
        "kernelspec": {
            "display_name": "Python",
            "language": "python3",
            "name": "python3"
            },
        "file_extension": ".py",
    },
    "julia": {
        "kernelspec": {
            "display_name": "Julia 0.6.2",
            "language": "julia",
            "name": "julia-0.6"
            },
        "file_extension": ".jl"
    }
}

[mmcky]

Yes - this option is used to write the metadata to the jupyter notebook for language and kernel selection.

[mmcky]

@AakashGfude I think for the time being we should just update the default value to be python as that is the default lang in sphinx and see if we can infer values at a later date. I propose we add

jupyter_kernels = {
    "python3": {
        "kernelspec": {
            "display_name": "Python",
            "language": "python3",
            "name": "python3"
            },
        "file_extension": ".py",
    },
}

as the default value for the option, then it is not mandatory to specify in conf.py and notebooks won't be build without the kernel information provided.