[coming features] Refactor of project structure

[martinezmatias]

Currently, we project has the code related to features extraction splitted in two packages:
fr.inria.coming.codefeatures and fr.inria.prophet4j.
All code related to features inside 'fr.inria.prophet4j.should be moved tofr.inria.coming.codefeatures. Moreover, we should do a cleaning of the fr.inria.prophet4j. ` package.
For example, there are some classes that have duplicate features (and old set of S4R features)
S4RFeature, S4ROFeature.

Then, we need to decide where to put all the code from fr.inria.prophet4j.that we dont move to fr.inria.coming.codefeatures (for example learner, demo)

[monperrus]

cleaned feature code in #266 and #267

now we only have EnhancedFeatures