Update Classical PSHA-based Calculator algorithm

[matley]

Currently, the classical psha-based calculator assumes that a vulnerability function is strictly increasing.

However, the user should be able also to submit non-decreasing vulnerability function, but still be able to use the calculator.

In such case, as the vulnerability function is defined point-wise we just take into account the first intensity measure level and the first covariance coefficient given in input.

E.g.

Data in input:
imls = 0, 0.1, 0.2, 0.3
mean loss ratios = 0, 0.1, 0.1, 0.2
covs = 0, 0, 0.001, 0.002

Data considered:
imls = 0, 0.1, 0.3
mean loss ratios = 0, 0.1, 0.2
covs = 0, 0, 0.002

[matley (matley)]

Pull request: https://github.com/gem/oq-risklib/pull/50/

[Lars Butler (lars-butler)]

Related bug (changes required in oq-engine): https://bugs.launchpad.net/oq-engine/+bug/1166252