The disk-disk intersection algorithm was actually written in a way that
it works for any planar geometries in 3d space. Therefore, we outsource
the algorithm in a generic header that can be reused also in other
algorithms.

the algorithm we used for disk-line intersections so far is actually,
apart from very minor modifications, generic enough to be used for all
planar and bounded two-dimensional geometries in 3d space. Therefore, we
outsource it to a separate header where it can be reused in other
algorithms.

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