fvOptionsAdjoint was needlessly duplicating a lot of the functionality
of fvOptions in order to add an interface for computing sensitivity
contributions emerging from fvOptions. To reduce this code duplication:

- fvOptionsAdjoint was removed
- the corresponding sensitivity contributions have moved to fvOptions through
  virtual functions (returning a zero contribution in the base so
  backwards compatibility is retained)
- all sensitivity classes that were using fvOptionsAdjoint have been
  modified appropriately
- all adjoint solvers are now grabbing a reference to an fvOptionList
  from the database instead of constructing an fvOptionsAdjointList

Hence, all fvOptions contributions to the adjoint equations
or the sensitivity derivatives can be given through system/fvOptions,
removing the need for separate sub-dictionaries within optimisationDict.