message: Mention GradientInterpolator and IdentityInterpolator in CHANGELOG.
diff: Version 4.2.1 (development)
  ===========================
  +- Added matrix-free GPU-enabled implementations of GradientInterpolator and
  + IdentityInterpolator.
  +
  - Added interface to MUMPS direct solver. Its usage is demonstrated in ex25p.
  See http://mumps.enseeiht.fr/ for more details. Supported versions >= 5.1.1.
  