Output for general field return type.

- reorganize daclaration of OutputTime in output_time.hh
  leaving just public interface in public section and move
  it to beginning
- reimplement compute_field_data:
   - make it proper member of OutputTime class
   - use modified OutputData class template to compute values for any
     field return value

- simplify output classes for particular formats, try to support vector
  and tensor data, need some tests

- rename Output::Time:RefType to DiscreteSpace
- add const at() method to FieldValues
- darcy_flow ... add missing include of xio.h