ENH: more consistent handling/documentation of 'value' entry (#2703)

- for boundary conditions such as uniformFixed, uniformMixed etc the
  optional 'value' entry (optional) is used for the initial values and
  restarts.  Otherwise the various Function1 or PatchFunction1 entries
  are evaluated and used determine the boundary condition values.

  In most cases this is OK, but in some case such coded or expression
  entries with references to other fields it can be problematic since
  they may reference fields (eg, phi) that have not yet been created.

  For these cases the 'value' entry will be needed: documentation
  updated accordingly.

STYLE: eliminate some unneeded/unused declaration headers