Include alternative parametrizations for Weibull and Exponential hazard potentials

[Iñigo Bermejo]

There are two different parametrizations for both Weibull and Exponential hazard potentials. Right now only one of them is implemented: the Weibull parametrization is the one used in the Centre for Health Economics of the University of York and STATA. This parametrization is different from the one specified in the ProbModelXML document. The same happens for the exponential potential, which is a special case of the Weibull distribution.

Implement the parametrization specified in the ProbModelXML document for both potentials and somehow inform the user of both parametrizations inside OpenMarkov.