Backwards-compatibility fixes

This series of patches treats four separate backwards-compatibility issues that were introduced in Kwant 1.4. See the whatsnew file for the individual issues.

One restore backwards compatibility even further, but IMHO this would not be worth the extra work and/or code complexity. When judging how far to go, I've been following the rule that code that we have published prominently should not suddenly stop working from one version of Kwant to another without that there's first a deprecation warning.