Improve sdf generation performance

[basicNew]

Fixes #12

Basically just caching the Jinja template so it doesn't need to parse it over and over. Time has drastically changed:

$ time terminus/run_generator.py --builder=ProceduralCityBuilder --destination=hey.world --parameters filename=../procedural_city_generation/procedural_city_generation/temp/mycity

Before:

real 0m11.854s
user 0m11.768s
sys  0m0.024s

After:

real 0m0.464s
user 0m0.408s
sys  0m0.052s

@ernestmc @garyservin the only important thing to remember is that now the template() method lives on the class side, so you should define it like:

    @classmethod
    def template(cls):
        ...

If we happen to need to redefine the template at the instance level (can't think why, but in any case) you can just override the instance method jinja_template(self). You will however loose the caching.