- both versions handle the same input words.
  Only need the <Switch> version when the destination variable is
  also a Switch and we need to output the word later.

This is controlled by providing a list of the fields to be cached in the "cache"
sub-dictionary of fvSolution.

Debug information about the caching is printed when the solution::debug switch
is on.

There are still a couple of issues to do with the naming of gradients used in
corrected snGrads and limited interpolation schemes that need to be resolved
but these are no different to previously and hence not urgent.

- PackedList comments
- solution.C verbosity about new solver syntax on master only

- partial revert for commit d21869b5
  * only add extra newlines for a top-level dictionary that is output as such

- make "#inputMode merge" the default instead of "#inputMode error"
  * this corresponds to a very common usage case

equivalent to the 'none' entry in fvSchemes and relaxation factors must be
provided for all fields which might require one.  If it is set to a value
between 0 and 1 this value is used as the default relaxation factor.

  - can now use dictionary substitutions and regular expressions in
    system/fvSolution

  - foamUpgradeFvSolution application to convert system/fvSolution
    (with -test option)

motion solver syntax left as-is.