Drop the `r` channel

Use conda-forge's blas package w/openblas

As conda-forge does not use the `nomkl` package, but instead uses the
`blas` package to select BLAS versions, install the `blas` package and
select the `openblas` implementation.

The `r` channel has been considered part of `defaults` since `conda` version `4.3.0`. So it should already be taken into consideration by `conda` installs without having to explicitly add the channel. Further the `conda-forge` channel has incorporated an ever growing, healthy stack of R packages. As such it appears all of the R packages used in this stack now come from `conda-forge` and not `defaults`. Given all of this, it seems safe to drop the `r` channel from explicit addition to the channel list.

ReadTheDocs site, User Guide sections

Make extracted spark files root id

base image on ubuntu:xenial-20180228

I can't seem to configure the Jupyter Albaster theme correctly
and will reach out for help later.

Update tensorflow-notebook readme with distributed mode instructions

Add TEST_ONLY_BUILD arg

Fix netcat, pin tornado 4.x for R + zmq issues, update various R libs

Disable datascience-notebook precompilation

Add netcat