Keep unchanged sparsity pattern

Add:

• bool argument to BiLinearForm::init to force pattern update
• BiLinearForm::setSymmetryStructure function
• MatrixFacade::init() setting all matrix entries to zero while keeping the pattern intact
• Implementations MatrixBackend::init()

Change:

• SymmetryStructure is now provided to the SparsityPattern and no longer used in MatrixFacade::init
• SparsityPattern::init calls are replaced by constructor calls
• BiLinearForm::init either rebuilds the pattern or just sets entries to zero, depending on a flag
• BiLinearForm sets a pattern rebuild flag instead of immediately rebuilding it when observing a basis change
• BiLinearForm also tracks changes to its operators to set the pattern rebuild flag
• SymmetryStructure is now supposed to be set by the user after adding operators