Added wavelet tree matrix for integer alphabets.
Stricter check in select.
Generic interval_symbols implementation + add WM to tests
Finished wavelet matrix

* Added range_search_2d method to wm_int
* Included wm_int in the tests
* Simplified test framework
Fixed clang compiler warning.
Offset caching in range_search_2d.
Merge pull request #157 from simongog/wavelet_matrix

Wavelet matrix