various changes towards dpllt integration of the congruence solver
preliminary DPLL(T) implementation done, all but one compile error fixed
changed handling of literal ids. more general now, shouldn't be a big influence on performance and is extensible. does not compile, because change to propositional skeleton necessary in order to tell theory and propositional literals apart
test6 passing with Unsat result, but matcherror with rest
changed handling of literal and literal id. literal id isn't thread safe and a
setting to build.sbt stopping sbt from running tests in parallel had to be
added. all tests are running. added DPLLTSolver and TheorySolver, for DPLLT
implementation.