Mixed precision functionality

This issue is for discussions related to adding mixed precision capabilities to libCEED, including:

- Which user-facing functions should have mixed precision capability?
- How should the libCEED interface support this (what will the user need to do)?
- What combinations should be available (e.g. mix-and-match input and output precisions, ability to decouple input/output precision from precision of computations, ...)
- Potential for backend specializations, and how this will fit with the interface changes

An idea from @jedbrown was to handle precision conversions through the `CeedVector` objects, which could be modified to store multiple pointers for different precisions, along with a tag to indicate which precision that vector's data currently uses.