Fixed precision

Changed (some of) the exporters to write the floats in scientific notation with a fixed precision of 10 digits.
With this, it is possible to compare directly the expected output files, which is extremely convenient, since one can use diff/meld and similar tools. This is a work in view of moving the QA tests from the engine into commonlib: the goal is to make sure that the commonib calculators give the same results of the engine calculators. See https://bugs.launchpad.net/oq-engine/+bug/1374281.

The tests are green: https://ci.openquake.org/job/zdevel_oq-commonlib/194/