[geometry] make all geometries default-constructible

See merge request tools/frackit!89

This could be usefol, for example, in the case that one passes
geometries as references to a function in which they are actually
created. At the point of creating these objects in order to pass them to
a function one might not have enough data/information to actually
instantiate them.

Fix/clang compilation issues

See merge request tools/frackit!88

This led to linker errors using clang, removing it made it work with
both clang and gcc.

This failed to compile using clang.

Fix/entity network virtual ctor

See merge request tools/frackit!87

[cmake] fix c++ standard in release flags

See merge request tools/frackit!86

Feature/improve occ utils

See merge request tools/frackit!85

[sampling][box] fix docu

See merge request tools/frackit!84

[geometry][hollowcyl] check given ctor arguments

See merge request tools/frackit!82

[test] fix include for std::runtime_error

See merge request tools/frackit!83

[cmake][testmacro] mention dune

See merge request tools/frackit!81

[example2] add section on constraints on confined entities

See merge request tools/frackit!80

[readme] link python bindings MR

See merge request tools/frackit!79

[geometry][direction] export dimensionalities

See merge request tools/frackit!78

[hollowcylinder] rename mantle -> lateralFace

See merge request tools/frackit!77

Feature/improve samplers

See merge request tools/frackit!73

Feature/improve cmakelists

See merge request tools/frackit!75

[geom][box] add missing include and alias

See merge request tools/frackit!71

Feature/hollow cylinder

See merge request tools/frackit!72

Feature/face face intersection angle

See merge request tools/frackit!76