Feature/accessibility indicator

This adds an accessibility calculator for total population and low income.

The following changes are also relevant:
 - stop buffers are now initially calculated in postgres and then combined using JTS' `CascadedPolygonUnion` for speed/efficiency
 - A new demographics param is added
 - Parameters now live in their own directory as do calculators

Working on getting tests to added and running, but the rest of the code is open for comment.