update traversal and tree container based on if constexpr feature

See merge request !182

use std::void_t instead of Dune::void_t

See merge request !175

Remove check for class template argument deduction

See merge request !179

remove check for fold expressions

See merge request !181

remove check for auto template parameters

See merge request !180

Add mkdocs documentation

See merge request !160

use std::is_invocable instead of Dune::Std::is_callable

See merge request !178

use if constexpr instead of Hybrid::ifElse

See merge request !177

use concept aliases in BoundaryConditions

See merge request !170

use value template instead of ::value

See merge request !176

use std::apply instead of Dune backport

See merge request !173

use std::optional instead of Dune::Std::optional

See merge request !172

cleanup boundary conditions

See merge request !99

Add documentation to boundary conditions
Correct DirichletBC::fillBC to use column basis for solution

introduce aliases for dune-function concepts

See merge request !169

remove old compilers with c++14 from gitlab-ci

See merge request !167

use explicit refinement Intervals in subsampling vtk writer

See merge request !168

Replace explicit type LocalView by template with concept check

See merge request !164

Use 2.6 Dune::Factorial

See merge request !165

Replace explicit type Dune::DefaultGlobalBasis in ctor argument with concept check

See merge request !162