ENH: introduced unsteady adjoint functionality

- The unsteady adjoint equations are integrated backwards in time. Since
  each adjoint time-step requires the primal solution of that time-step
  to be known, schemes for managing the storage/retrieval of the entire
  flow series are necessary. These are implemented through the
  primalStorage class and its derived ones. The latter manipulate a new
  class of fields, called compressedGeometricFields, which provide hooks
  for compressing/decompressing a field during the time integration of
  the primal/adjoint equations. The method used for
  compressing/decompressing is run-time selectable.
- The current commit provides the shortGeometricField implementation
  which avoids the storage of patchFields that can be retrieved from the
  internalField (e.g. coupled, zeroGradient, symmetry, etc) , to cut on
  the storage requirements. More elaborate compression approaches will
  be included in the future, during the exaFoam project.
- Two primalStorage options are included: compressedFullStorage and
  binomialCheckPointing.
    - compressedFullStorage stores the entire flow time-series,
      potentially by compressing each time-step (only the
      above-mentioned short approach is available for the moment).
    - binomialCheckPointing is based on the homonymous algorithm
      found in

      \verbatim
          Wang, Q., Moin, P., & Iaccarino, G..
          Minimal Repetition Dynamic Checkpointing Algorithm for
          Unsteady Adjoint Calculation (2009).
          SIAM Journal on Scientific Computing, 31(4), 2549-2567.
          10.1137/080727890,
      \endverbatim

      which stores the solution of the flow equations in a predefined
      number of time-steps, named checkpoints. During the
      backwards-in-time integration of the adjoint equations, if the
      primal solution at a certain time-step is not available, it is
      retrieved by re-computing the primal flow field starting from the
      closest checkpoint. Checkpoints are optimally distributed
      throughout the time-series to invoke the least number of flow
      recomputations during the backwards-in-time solution of the
      adjoint equations. Binomial checkpointing is the current state of
      the art though its re-computation cost frequently amounts for an
      extra solution of the flow equations in medium-to-large cases.
- The adjoint to the PISO and PIMPLE solvers, along with their
  solverControl variants, are additionally included.
- Objective functions are integrated in time, through appropriate
  entries in the dictionaries defining them.

Authored by Andreas Margetis and reviewed by Vaggelis Papoutsis, with
earlier contributions from Dr. Ioannis Kavvadias.