Provide function for distance calculation (spherical)

[Muharem Hrnjadovic]

We need to be able to calculate the shortest distance over the earth’s surface for two given points.

Please see http://www.movable-type.co.uk/scripts/latlong.html for details.

[Lars Butler (lars-butler)]

A little bit of context for this bug:

The results produced by the Disaggregation are 5-dimensional matrices (Lat x Lon x Magnitude x Epsilon x TectonicRegionType). There are many options for producing subsets of this data. Some examples include: Lat-Lon-Magnitude, Lat-Lon-Epsilon, and Magnitude-Distance.

In the case of results involving Distance, the Distance must be calculated between a Lat/Lon value (in the matrix) and another site of interest. (Note: In this case, both locations have a depth of 0.0, indicating the surface projection of a 3D geographical point.)

OpenSHA currently provides some function(s) to calculate this distance. However, we plan to extract subsets from the full Disaggregation matrix asynchronously in Python using Celery tasks. Therefor, we need to implement the equivalent functionality on the Python side, and also test it against the OpenSHA code to make sure both methods produce the same results. (I suspect that the Shapely [1] module provides this functionality already, but we'll have to check. Either way, we still need to compare the results with those produced by OpenSHA.)

[1] http://pypi.python.org/pypi/Shapely