optimize bbox_from_point

[gboeing]

resolves #540

[coveralls]

Coverage increased (+0.2%) to 92.697% when pulling f874ab5 on fix into 4a8aaa0 on master.

[gboeing]

As a simple test:

import osmnx as ox
dist = 500
point = 37, -117
north, south, east, west = ox.utils_geo.bbox_from_point(point, dist=dist, project_utm=True)
print((north - south) / 2 - dist) #-1.177
print((east - west) / 2 - dist) #0.997
print(ox.utils_geo.bbox_to_poly(north, south, east, west).area) #999636

This desired 1 km^2 projected bounding box is within ~1 meter of expected linear dimensions and has area of 0.9996 km^2.

The new implementation of ox.utils_geo.bbox_from_point is several thousand times faster than the old.