ENH: add global (serial) path accessor to IOobject (#3007)

- new methods added to IOobject to ease mixed (serial vs parallel)
  file locations. Some redirect to Time, others are defined for
  IOobject only.

    | "normal" (serial/parallel) | "global" (serial locations) |
    | ---------------------------|-----------------------------|
    | caseName()                 | globalCaseName()            |
    | path()                     | globalPath()          *new* |
    | path(...)                  | globalPath(...)       *new* |
    | objectPath()               | globalObjectPath()    *new* |