Add warm-up period simulated flows in OutputsModel

SD models need the upstream flows simulated during the end of the warning period in order to be correctly initialised.

Simulated flows during the warm-up period are part of the GR fortran function outputs. Consequently they can be add in the outputs of the RunModel_GR* functions.

We propose to add a new item in OutputsModel called "WarmUpQsim" containing this simulated flow.

Unlike the other outputs this one will have the same length as IndPeriod_Run. In anticipation of #89 (closed) and the future release of the '[.OutputsModel' method, this new item should have an attribute specifying that it should not be processed by such methods. This attribute could consist to add a class named "WarmUpOutputsModelItem".