Multiple RHS support (+ Solve with HPDDM)

src/problem/Formulation.cpp

src/problem/Formulation.h

src/system/Solver.cpp

src/system/Solver.h

src/term/Assembler.cpp

src/term/Assembler.h

src/term/LinearTerm.cpp

src/term/LinearTerm.h

src/term/LinearTermInterface.cpp

src/term/LinearTermInterface.h