Calculate yup/ydown on demand

[johnomotani]

Call calcYUpDown wherever yup/ydown fields are required (it now does nothing if yup/ydown are already present and valid), instead of in Mesh::communicate().

• makes yup_field and ydown_field members of Field3D mutable
• adds mutable bool has_yup_ydown to Field3D, so that in Solver::load_vars() we can mark the yup/ydown fields as invalid without deleting them
• new Field3D& ynextMutable(int dir) const which can be used to get non-const versions of the mutable yup_field and ydown_field from a const Field3D
• calcYUpDown() methods take a const Field3D as argument

[ZedThree]

I don't think this is the correct thing to do. calcYupYdown is changing the logical state of a field, therefore it should not be done on a const Field3D.

I think this would only be acceptable if the parallel slices were completely hidden from the user, i.e. they were an implementation detail.

Having a bool hasValidParallelSlices is not a bad idea, but we'd have to be very rigorous about keeping it correct. All the arithmetic-assignment operators would need to mark it as invalid for instance.

[johnomotani]

Think @ZedThree is right. For example if a user wanted to force a function to use the field-aligned version instead of yup/ydown while using ShiftedMetric everywhere else, this implementation would prevent them from doing so because the const Field3D would be modified 👎

Closing this, but I'll take over the bool hasValidParallelSlices idea to #1176.