check dimensions for multiple setters

[hexaeder]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

fixes #82

Technically breaking? But I personally would consider the old behaviour a bug.

[hexaeder]

Instead of changing from map to broadcast, it would also be fine to check the sizes explicitly

if size(ms.setters) != size(val)
    throw(DimensionMismatch("Cannot set variables of size $(size(ms.setters)) to values of size $(size(val))."))
end

My point about the bug is the silent dropping of setters/values rather than the missing broadcast behavior.