Edits to make novice R lesson 01 more similar to novice Python lesson 01.

I have made edits so that the first R lesson more closely follows the Python lesson. It covers all the same topics save two:

  + Loading external packages - omitted because none of the operations in the lesson require loading an external package.
  + How to display three plots side-by-side - omitted because this is more complicated in R. This would require discussing how to set and reset `par` options, which is not a beginner topic.

Here's how you can quickly compare the two rendered files:

```
# clone bc repo
git clone git@github.com:swcarpentry/bc.git
# create and switch to new branch
git checkout -b review-pr
# add remote
git remote add jdb git@github.com:jdblischak/bc.git
# pull changes from my branch
git pull jdb 01-starting-with-data
# build site locally
make site
```
Then compare the rendered files  `_site/novice/r/01-starting-with-data.html` and `_site/novice/python/01-numpy.html`.
do you also want to mention matrix here?

the actual mean, median, and sd calls are on a matrix

but they're converted from a data frame, so I'm not sure...