Improve the documentation

@guillaume.thirel:

In the RunModel_DA() help, I do not understand this sentence: "When accounting for the uncertainty in model inputs (see InputsPert), the set discharge simulations are driven by perfect meteorological forecasts, which are generated according to the methodology proposed by Clark et al (2008) (see CreateInputsPerturb for details). "

In the example of RunModel_DA, also put the obs in the range calculation, because your example takes it out of the graph! ylim = range(rangeQsimEns),

I admit that for this example, I would have put the PF rather than the EnKF, because that is what is recommended in the article.

@gaia.piazzi, "temporal decorrelation length of 1 day for rainfall and 2 days for potential evapotranspiration": how should it be adapted for hourly time steps?