Add a new type of generator : rectangular function

[crevoisier]

inputs :

• required : begin date DT1
• required : end date DT2
• required : value Val (all openfluid::core::Value allowed)
• used (?) : value Zero

outputs :

• Val if current date DT between DT1 and DT2 ( DT1 <= DT <= DT2)
• Zero else (or no value)

To investigate :

• can Zero have default value for all openfluid::core::Value ?
• is it possible / useful to allow the generator not to produce a variable outside [DT1:DT2] if Zero is not provided (excepted for initialization : which value in this case) ?
• if Zero is not provided, how to distinguish no value produced and default zero value produced ?