minor cleanup of turbulence models

- added constructor dictionary(const dictionary*) that also handles NULL
  pointers and makes it convenient to construct from a possibly nonexistent
  sub-dictionary:
  eg,
      dictionary dict2(dict1.subDictPtr("someDict"));

- make some of the turbulence Coeffs sub-dictionary optional.
  Their contents are all 'lookupOrAddDefault' anyhow.

- in turbulentMixingLength BCs, skip namespace qualifier in template
  (eg, <RASModel> vs. <compressible::RASModel>)

- change comments from 'turbulenceProperties' to RASProperties/LESProperties

- consistency between compressible/incompressible - no separate file for
  'New' selector etc

- consistency in accessing the model coefficients.
  Use method coeffDict() for const access.
  Use protected data member coeffDict_ for read/write access.

- document model coefficients in etc/constant/RASProperties.
  Need the same for LESProperties before we can prune these from the
  tutorials.