Saving dimacs to xml ignores features not occurring in constraints

[meinicke]

Prerequisitives

• Eclipse Version: 2018-12
• FeatureIDE-Version: 3.5
• Operating system: WIN 10
• Java runtime used to run eclipse: 1.8
• FeatureIDE Composer: Feature Modeling
• Git branch/Commit-ID: N/A
• Java compiler used to build the FeatureIDE plugin: N/A

Issue description

When saving (probably when loading directly) dimacs files to xml, FEatureIDE ignores features that do no appear in constraints.
The dimacs parser should use all the features defined in the first rows befor the "p cnf x y" entry, they are meant to be optional.

As shown below, the dimacs file declares 6 features, but the xml has only 4, the 4 that appear in constraints.

The dimacs file I used:
c 1 base
c 2 weight
c 3 empty
c 4 executivefloor
c 5 twothirdsfull
c 6 overloaded
p cnf 6 3
-6 2 0
-5 2 0
1 0