add cleanup method to graph, use node v11, fix tests

[mguida22]

Hello!

We've been using sharedstreets-js at RideReport for a couple months now, and it's been a great help to build on the shared streets reference system! Thanks all of the work that has gone into this!

I've included a few changes here that we had to make to get sharedstreets-js working for our use cases. Feel free to use these changes, modify them, or ignore them.

• add a note that sharedstreets-js requires node v11 due to it's dependency on OSRM
• fix the ordering of expect statements within test_core
• add a cleanup method to the graph

---

The only real functionality change is adding a cleanup method.

When a graph is built, it creates a LevelDB database to use. This node process puts a lock on the database, but doesn't release the lock until the node process ends (usually the node script finishes running, or if there's some error). This causes an issue when the same node process wants to get a new graph instance, built off of the same inputs. When a second graph is built with the same input parameters, it attempts to use the existing LevelDB, but sees the lock and throws an error.

/Users/mike/oss/sharedstreets-js/node_modules/levelup/lib/levelup.js:119
      return callback(new OpenError(err))
                      ^
OpenError: IO error: lock /Users/mike/.shst/cache/graphs/e8958d1c-1464-373e-b6b8-5a4959e309d2/db/LOCK: already held by process
    at /Users/mike/oss/sharedstreets-js/node_modules/levelup/lib/levelup.js:119:23
    at /Users/mike/oss/sharedstreets-js/node_modules/abstract-leveldown/abstract-leveldown.js:38:14
    at /Users/mike/oss/sharedstreets-js/node_modules/deferred-leveldown/deferred-leveldown.js:31:21
    at /Users/mike/oss/sharedstreets-js/node_modules/abstract-leveldown/abstract-leveldown.js:38:14
Emitted 'error' event at:
    at /Users/mike/oss/sharedstreets-js/node_modules/levelup/lib/levelup.js:60:19
    at /Users/mike/oss/sharedstreets-js/node_modules/levelup/lib/levelup.js:119:14
    at ers/mike/oss/sharedstreets-js/node_modules/abstract-leveldown/abstract-leveldown.js:38:14
    at /Users/mike/oss/sharedstreets-js/node_modules/deferred-leveldown/deferred-leveldown.js:31:21
    at /Users/mike/oss/sharedstreets-js/node_modules/abstract-leveldown/abstract-leveldown.js:38:14

This issue can be avoided by calling .close() on an open db connection when it's done being used.

These changes expose a .cleanup() method on the Graph, that closes the LevelDB connection. Once .cleanup() is called on an instance, a new instance can be created that accesses the same data. This allows for one continuous process to create and cleanup multiple sharedstreets Graphs.

(a lot of whitespace changes got included by my editor, let me know if you want me to remove them)

[mguida22]

This is the main change to add a .cleanup method