restrict Capillary.gradient to vectors to avoid confusions

[chrisbrahms]

this is mainly to fix some issues further down the line with using some processing functions which parse the arguments to prop_capillary based on type. needs a better long-term fix but this way at least that throws the correct error.

[jtravs]

This is a breaking change, in that I often use a Tuple for this. Is there no other way around this?

[chrisbrahms]

I should've been more specific--the restriction is only for multi-point gradients. For simple input-output gradients you should continue to use a Tuple anyway. Is that still a problem?

The issue comes just in parsing the arguments to prop_capillary (from a string) when re-creating the modes from a simulation. the ideal solution is to make that more robust in and of itself by saving more information about the modes in a "native" format. but this will at least throw an error in the correct place until I can get to that (haha).

[jtravs]

No, I always use Tuples even for multi-point gradients. But it doesn't really matter, I am probably the only one and I can fix it.