Collect small issues for forecast classes

[chahank]

Here we collect small non critical issues to be addressed for the new forecast classes.

• Add coment in the forecast base class to explain how the inheritance of the ImpactForecast and HazardForecast classes work on the level of the super.
• Add check of array size in the inits of ImpactForecast and HazardForecast to make sure data is coherent.
• Check that the frequency of the events is equal to 1/#ensemble memebers in the HazardForecast Class.
• The init of the ImpactForecast class should set the eai_exp to an empty array of length equal to coord_exp and aai_agg to a non-definit value. This is in addition to having a property for these attributes that would return the correct values, namely the eai_exp and the aai_agg for each lead time.