Feature/fix sum operator

This is a suggestion on how to cleanup the InstationarySumLocalOperator and WeightedSumLocalOperator local operators.

@rhess please have a look...

I cleaned up the implementation in such a way that we have free functions, which wrap the LocalAssemblerCallSwitch, e.g. LocalOperatorApply::jacobianBoundary(lop, ig, lfsu_s, x_s, lfsv_s, mat_ss);. This allows to cleanup and unify the implementation of InstationarySumLocalOperator and WeightedSumLocalOperator.

I further adds a default value to the isLinear parameter of LocalAssemblerCallSwitch.

I suggest to

• move this modified interface of LocalAssemblerCallSwitch to an Impl namespace
• change the assembler to use the new free functions
• add a deprecated class LocalAssemblerCallSwitch which forwards to Impl::LocalAssemblerCallSwitch

fixes #136 (closed)