Expose neglect_steady in transient Model.headalongline

[MattBrst]

Forwards the existing head() neglect_steady flag through headalongline so transient-only heads along a line can be computed once a steady model is linked via Model(steady=...).

Should close issue #180

[mbakker7]

This is an interesting thought. If I understand it correctly, what you propose is that when a steady model is added to a transient model with Model(steady=...), then you still want to be able to compute the headalongline for just the steady solution. If we implement a neglect_steady option, then we have to do that for a bunch of functions (head, contour, trace, etc.). So I am not sure this is the way to go.

What we are thinking off for the future is to have one "combined" model that includes a steady model and a transient model. For that general model it may indeed be useful to be able to get "only" steady or "only" transient heads. Although I see this was already merged into dev, we may want to re-consider. (And for the future, it may be good to discuss these new additions a bit before submitting a PR, so you don't do unnecessary work).

[MattBrst]

As written in #180, for me it is OK to revert this PR.