Boundaries of the (b-type) deflector

- The deflector has a special shape
- This should be implemented as a boundary (contrary to just using the `NaN` values as boundary)
- For this one needs the analytical description of the boundary of the deflector and then detect whether the molecule is _inside_ this boundary
- The detector _probably_ is a (3D extended) rectangle _minus_ two ellipses -> the particle hit the boundary if it is outside the rectangle _or_ the ellipses
- At some point other deflectors (e.g. a-type) should be supported as well