Cleanup of risklib, in particular of the calculators and curve.Curve

This pull request contains two major things:

1. now all the calculators are classes with a __call__ method; this ensures consistency, pickeability and extensibility (before some calculators were classes and some were closures).

2. I have cleaned up the Curve class and remove unused or useless functionality; moreover now
the interpolation is done only once, not every time ordinate_for is called.

This is a first step preliminary to https://bugs.launchpad.net/openquake/+bug/1081937