Merge branch 'fix/CHANGELOG' into 'master'

CHANGELOG.md: Latest change belongs to 2.10

See merge request !70

CHANGELOG.md

 # Master (will become release 2.10)
 
+- A new solver `ProximalNewtonSolver` is added which solves non-smooth minimization problems.
 
 # Release 2.9
 
-- A new solver `ProximalNewtonSolver` is added which solves non-smooth minimization problems.
-
 - The internal matrix of the`EnergyNorm` can now be accessed by `getMatrix()`.
 
 - `codespell` spell checker is now active for automated spell checking in the Gitlab CI.