Cherry pick from feature-staticPartitioning to get all but the staticPartitionner.

Enough features for a release. Need to clean before releasing however.
Clean
Code cleaning
update changelog
fix conflict
Missing documentation
minor cleaning
Prepare the code for release 0.31
[maven-release-plugin] prepare release btrplace-solver-0.31
[maven-release-plugin] prepare for next development iteration
integrate the development version 0.31-SNAPSHOT generated by maven
code prepared for development version 0.31-SNAPSHOT
next development version
Merge branch 'develop' into feature-staticPartitioning
Put the element building part out of the model.

Provide a naive and a thread-safe element builders. Second
is necessary for paralelized resolutions of instances.
Missing javadoc
Refactor and test Splitters