Undocumented and unused: getlowerorder

[KnutAM]

We have the internal function getlowerorder which seems to be legacy (at least I don't see that we use it internally, and it is not documented in devdocs).

Only a few interpolations are supported. Would be good to either remove (since we don't use it), or (dev)document it at least. Keeping it leads to extra code for implementing new interpolations...