sql: Add a submodule with Greg Wilson's SQL via IPython notebooks

sql: Add a submodule with Greg Wilson's SQL via IPython notebooks

We're linking to the novice SQL lessons cherry-picked from
swcarpentry/bc's master branch [1].  The usual layout is
$subject/$tool, so maybe this should be sql/sqlite?  I find it hard to
imagine folks teaching SQL to novices without using SQLite though, so
I've skilled the per-tool repository.  It will be easy to add the
$tool directory if we *do* get someone teaching SQL with another tool.

[1]: https://github.com/swcarpentry/bc