api: allow configuration of dataset locations

Add the ability for app.config to override both the elevation and wind
dataset locations. The default behaviour is unchanged.

(This is to allow sensible testing on one's own machine without
hard-coding paths.)
api: add tawhiri-webapp utility

Use Flask-Script to provide a CLI to run a development webserver version
of tawhiri. Allows the configuration code from the previous commit to
actually be used.
remove test_api.py

The functionality is replaced (and extended) by tawhiri-webapp.
api: use correct app.config idiom for defaults

I don't know what I was thinking earlier. ``app.config.get()`` does what
I want here.
Merge pull request #86 from rjw57/api-configuration

Add configuration for dataset locations