fix a critical bug in the set_box_info method of BoxMap - #7

This fix takes the time complexity of the
 algorithm down to O(n^2) from an estimated O(2^n).
Also did a lot op optimisations to the code,
 increasing performance by a large margin.
Some benchmarks follow:
- In both the optimised and unoptimised versions
 of the code the fix has been applied.
- All test were done on an AMD FX-6350 @ 4.5Ghz,
 with the same test case for both versions.
- The code was built using the supplied cmake build,
 using the release build configuration.

Test size | total insertions | unoptimised | optimised
50        | 10000            | 1610+-100ms | 195+-10ms
100       | 40000            | 33.2+-.1s   | 3.21+-.5s
150       | 90000            | 199+-3s     | 19.3+-.1s

From these tests it can be concluded that
 there is an average speedup of around 860%.
It can also be observed that the algorithm conforms
 reasonably well to the calculated O(n^2) complexity.