Improve performance of environmental suitability calculation

There are now 2 ways of calculating environmental suitability:

1. For a batch of disease occurrences, all for the same disease group, use GeoTools to read the raster into the middle tier and find the environmental suitability. Reading into the middle tier is slow (around 3 seconds on my machine), finding the environmental suitability is very fast (0.05 seconds).

2. (already implemented) For arbitrary disease occurrences coming from Data Acquisition, use PostGIS to find the environmental suitability. This avoids the time and memory overhead of reading the raster into the middle tier, particularly if there are multiple disease groups acquired. The calculation is moderately slow (0.6 seconds on my machine) - but we are only expecting around 150 HealthMap occurrences per day.