message: Update CHANGELOG with a bullet for MFEM_VERSION.
diff: Development version 3.3.1, not released
  =======================================
  +- Added a new macro, MFEM_VERSION, defined as a single integer of the form
  + (major*100 + minor)*100 + patch. The new convention is that an even number
  + (i.e. even patch number) denotes a "release" version, while an odd number
  + denotes a "development" version. See config/config.hpp.in.
  +
  - Add a block lower triangular preconditioner in using a matrix-free
  implementation, see class BlockLowerTriangularPreconditioner.
  