Add some algorithms for data transformation

 https://github.com/gem/qt-experiments/pull/22 should be merged in master first (and many new features currently present in master should be merged here)

Addresses https://bugs.launchpad.net/oq-svir/+bug/1269338 and https://bugs.launchpad.net/oq-svir/+bug/1269335

* Removed dependency from scipy. Ranking has been re-implemented, including all the different variants for coping with ties (that are present only in the most recent versions of scipy), and also the corresponding inverse methods have been implemented
* log10 transformation implemented (using numpy). The only difference with respect to the algorithm available in the built-in QGIS field calculator is that in case of zeros in the input we replace 0 with 1 before performing log10 (as it is a pretty common practice)
* simple quadratic transformation implemented, with "increasing" and "decreasing" variants and corresponding inverse methods
* Unittests implemented for every available variants (and inverse methods) of the algorithms