[localevaluation.elliptic] do not use default template arguments for traits
[localevaluation.swipdg]
* use common function for default_beta, inner_sigma and boundary_sigma
* prepare for specialization with additional diffusion tensor
* made static methods const
* put traits into internal namespace
[operators.elliptic-swipdg]
* use correct default_beta
* prepare for specialization with an additional diffusion tensor
[functionals.swipdg]
* use correct default_beta
* prepare for specialization with an additional diffusion tensor
[operators.elliptic-cg] remove duplicated code
[localevaluation.swipdg] added specialized variants to playground
[operators.elliptic-swipdg] added specialized variant to playground
[functionals.swipdg] added specialized variant to playground