Idea collection for a dofhandler interface

Here is a list of functions that could constitute a `AbstractDofHandler` interface in the future:
- `find_field(dh, fieldname)` --> return some type of field index
- `field_offset(dh, field_index)`
- `getfieldinterpolation(dh, field_index)`
- `getfielddim(dh, field_index)`
- `getfieldnames(dh)`

I don't think `getfielddims` and `getfieldinterpolations` are necessary in an interface, they could probably be removed to the point of being internals for `MixedDofHandler`.

I leave this issue here for future reference, I compiled the list while working on merging the dofhandlers - it needs some more investigation.