Feature/quad sampler

See merge request DennisGlaeser/frackit!26

Feature/abstract geom base class

See merge request DennisGlaeser/frackit!27

All geometries now inherit from an abstract base class, which currently
only defines the name() interface. Thus, currently the only requirement
that a geometry implementation must have is a name.

Feature/sampling with virtual base classes

See merge request DennisGlaeser/frackit!25

Geometry samplers can now be called to return a random geometry by
simply calling the () operator, without the need to externally compute
the point around which the geometry is to be created. This means that
the point sampler is now contained in the geometry sampler class. The
only currently available implementation (DiskSampler) has been modified
accordingly.

Feature/add make point sampler

See merge request DennisGlaeser/frackit!23

Feature/improve point sampler

See merge request DennisGlaeser/frackit!24

[exceptions] remove unneccessary std::string() around messages

See merge request DennisGlaeser/frackit!22

[test][io][gmsh] use confining domains in test

See merge request DennisGlaeser/frackit!21

Feature/improve geometry sampler

See merge request DennisGlaeser/frackit!18

Feature/entitynetwork use id class

See merge request DennisGlaeser/frackit!20