Added Uniform Hazard Spectra (UHS) post-processing to classical hazard calculator

[Lars Butler]

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General description:

Uniform Hazard Spectra are 2-dimensional curves extracted from hazard maps for a given:
  - PoE (Probability of Exceedance)
  - site
  - logic tree end branch or statistic (mean/quantile)

A UHS curve is constructed as follows:
  - abscissae (x-axis values) are SA (Spectral Acceleration) period values, sorted in ascending order
    - PGA is equivalent to an SA period == 0.0
  - ordinates (y-axis values) are IMLs extracted from a hazard map at a given location and PoE
  - example: http://pastie.org/6460997

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job.ini changes:

The following are required to implement this feature:
  - replace `poes_hazard_maps` with `poes`
    - this will be used by both maps and UHS
  - add `hazard_maps` (boolean)
  - add `uniform_hazard_spectra` (boolean)

If `uniform_hazard_spectra` == true, hazard maps will be computed no matter what. If `hazard_maps` == false, don't automatically export maps when --exports=xml is specified (the maps will be saved to the database, though).

`hazard_maps` == false and `uniform_hazard_spectra` == true is typical for single site studies, but not for regional calculations.

[Lars Butler (lars-butler)]

Related pull request, to do some initial cleanup before the main development of this feature: https://github.com/gem/oq-engine/pull/1088

[Lars Butler (lars-butler)]

Second pull request: https://github.com/gem/oq-engine/pull/1091

I still need to submit 1 more pull request which adds export functionality.

[Lars Butler (lars-butler)]

Export: https://github.com/gem/oq-engine/pull/1092