Add backbone detection and Bipartition & Elimination preprocessing

[AntonReinhard]

• add a Backbone preprocessing technique for use in preprocessing for model counting, finding the backbone of a formula
• add a bipartition and elimination preprocessing technique that finds definitions and eliminates variables that are defined in terms of others
• add a .clang_format file for consistent formatting everywhere
• fix problems when compiling with gcc11

[AntonReinhard]

See PR #22