Implements #90 for the case of sunpos (perhaps there are other cases that should also be implemented). Because of the ::DateTime type signature it is hard to misuse these methods (i.e. you can't accidentally pass a Julian date into them) so I think the cost of adding the implicit conversions is low. Especially compared to the convenience and risk of misusing explicit conversions. If AstroLib ever drops its dependency on Dates, these can move to a package extension. It's possible to implement this ever so slightly faster and with less code re-use to avoid allocating a vector to store the Julian dates I propose adding that optimization iff someone identifies this as a performance bottle neck.