Python Mo's Algorithm

[hannesfrank]

** Changes: **

This PR adds

• a generic python implementation for Mo's Algorithm and
• problem description, runtime and references to square_root_decomposition/README.md.

The implementation includes

• the generic algorithm,
• a sample implementation to compute the sum of the query-intervals (I could expand this with more interesting examples which could not be computed easily without the algorithm) and
• unittests.

Feedback is welcome and I'd love to improve this PR.