Release of G'MIC 2.5

[David_Tschumperle (G'MIC staff)]

Changelog for 2.5.6 , released on 2019/04/08.

New features:

• New filter Deformations / Warp [interactive] allows to create custom deformation with an interactive keypoint editor.
  
  

Improvements:

• Command map now accepts multi-channels input images, and output then an image with nb_channels(input)*nb_channels(colormap) channels.

Bug fixes:

• Fix 2.5.4->2.5.5 regression in command map.
• Fix compilation warnings that appeared with latest version of clang.