openfoam: avoid rescanning of time directories for decomposed

- after establishing the available times, based on the processor0
  directory contents, reuse these names and values for subsequent
  processor directories.

  The primary motivation is to avoid repetitive directory listings
  each time. The contents are expected to be consistent anyhow
  (at least according to the time-range output vector).

  Combined with the previous change for building the mesh time change
  caches, the startup for cases with a very large number of timesteps
  should be better.

BUG: set/get time information was inconsistent for sub-readers

- normally not visible when called from ParaView, but when called
  directly the SetTimeValue() method bypassed the sub-readers.