dependency graph for tensors

[martinjrobins]

currently dependencies between tensors is handled crudely, a tensor is dependent on either t, or u, or constant. This means that if you are calculating rhs, you might also calculate tensors that are only used to calculate out, for example.

Need a better way of tracking dependencies so that any function, rhs or out or other, only needs to calculate the tensors that it depends on

Could just keep all tensors in a vec, and then have a vec of indices for each tensor indicating its depenencies. Maybe a vec of (tensor_idx, tensor_elmt_idx) to indicate which element of which tensor it depends on