Clarify usage of fmi3GetNumberOfVariableDependencies and fmi3GetVariableDependencies

[chrbertsch]

... clarify for which variables one is allowed to call fmi3GetNumberOfVariableDependencies as well as which variable types will be included in the return from fmi3GetVariableDependencies. Is it all variables?

copied from #2024 (comment)

[chrbertsch]

FMI Design Webmeeting:

Klaus: I think you can call this for each variable listed in the model structure "initial unknown" or "output" section.