Consider combining or renaming `dynamics.phasespace`

[redeboer]

Since #292, there are two modules named phasespace: dynamics.phasespace (phase space factors) and kinematics.phasespace (phase space boundaries). There is a physical relation between the two, but this is not clear from the documentation.

Ideally, the modules are combined in a way that makes sense physically (see PDG Kinematics review in particular). Alternatively, simply rename to phsp_factor or something along those lines.