Merge branch 'feature/require-cmake-3.13' into 'master'

See merge request [!145]

  [!145]: Nonecore/dune-geometry/merge_requests/145

Merge branch 'issue/intel_const_constexpr' into 'master'

ref:core/dune-geometry\>

### Summary

Solve an intel-compiler related error, by replacing const by constexpr for
template parameter variables.

See merge request [!142]

  [!142]: gitlab.dune-project.org/core/dune-geometry/merge_requests/142

Merge branch 'bugfix/MissingAppend' into 'master'

See merge request [!141]

  [!141]: Nonecore/dune-geometry/merge_requests/141

Merge branch 'feature/addPythonBindings' into 'master'

See merge request [!140]

  [!140]: Nonecore/dune-geometry/merge_requests/140

Python bindings, default is OFF.

Merge branch 'add_using_directives' into 'master'

ref:core/dune-geometry\> This MR adds using directives for the use of
std::sqrt or std::abs. This is needed because other names in the namespace
Dune are probably found before the names in the namespace std.

See merge request [!139]

  [!139]: gitlab.dune-project.org/core/dune-geometry/merge_requests/139

fixed some bugs

Merge branch 'feature/remove-deprecated' into 'master'

See merge request [!136]

  [!136]: Nonecore/dune-geometry/merge_requests/136

The according code was depreated in Dune 2.6

Code was deprecated in Dune 2.6

Merge branch 'patch-1' into 'master'

ref:core/dune-geometry needs to be backported to releases/2.7

See merge request [!133]

  [!133]: gitlab.dune-project.org/core/dune-geometry/merge_requests/133

Merge branch 'feature/bump-to-2.8' into 'master'

See merge request [!132]

  [!132]: Nonecore/dune-geometry/merge_requests/132

Merge branch 'feature/add-geometrytype-id' into 'master'

ref:core/dune-geometry After thinking about what I proposed in [#17], I
realized that an index is probably not the right idea of thinking about this.
Instead, we can just do a 1-to-1 mapping of GeometryType to a canonical id, as
GeometryType itself is just a 8 byte struct that stores three integral values.

I've nested the id within GeometryType as GeometryType::Id here and added
implicit conversions between the two. The id type is a scoped enum to avoid
any future nastiness with implicit conversions to other integral types.

With this merge request, you can have a template with a GeometryType parameter
with just a tiny bit of overhead on the implementor's side:

    // note the additional "::Id"
    template<GeometryType::Id gt_>
    class Foo
    {
      // reconstruct the GeometryType
      static constexpr GeometryType gt = gt_;
    };

For the user, it looks like they can just plug in a GeometryType:

    Foo<GeometryTypes::triangle> foo;

This solves a major 2.6 headache in PDELab (see [#17],
[pdelab/dune-pdelab#102]), so I'd really like this to go into 2.6.

Closes [#17].

See merge request [!87]

  [#17]: gitlab.dune-project.org/NoneNone/issues/17
  [pdelab/dune-pdelab#102]: gitlab.dune-project.org/pdelab/dune-pdelab/issues/102
  [!87]: gitlab.dune-project.org/core/dune-geometry/merge_requests/87


Closes #17

While GeometryType is constexpr and can thus be used in a lot of compile time contexts, C++ does not
allow us to declare a template parameter of type GeometryType, as the standard only allows data
types that directly map to a built-in integral type for this purpose.

On the other hand, GeometryType itself is just a simple 64 bit struct that stores a few integral
values, so we can easily map it to an integral type.

This patch does just that by adding an opaque nested type GeometryType::Id that is implemented as a
scoped enum as well as implicit conversions between GeometryType and GeometryType::Id. By using a
scoped enum, we clearly tag the type as something which is not a normal number, because C++ requires
explicit conversion between scoped enums and other integral types.

The storage layout of the id is chosen identical to that of GeometryType, making conversion between
the two almost free.

When writing a template with a GeometryType parameter, just use the id instead and convert it back
into a GeometryType inside the template:

template<GeometryType::Id gt_>
class Foo
{
  static constexpr GeometryType gt = gt_;
};

Merge branch 'bugfix/multilin-geom' into 'master'

ref:core/dune-geometry This MR is simply setup to trigger the problem pointed
out in issue [#23].

See merge request [!125]

  [#23]: gitlab.dune-project.org/NoneNone/issues/23
  [!125]: gitlab.dune-project.org/core/dune-geometry/merge_requests/125