Property conversion and alternative initialisations for materials
The elastic properties of an isotropic, linear elastic solid can be described using different pairs of properties such as
(\lambda, \mu). It should be possible to convert the most commonly used pairs into each other.
Ideally, we have alternative ways to initialise these material classes, e.g.
solid = ElasticSolid.from_speed_of_sound(frequency, c_1, c_2, rho)
which will then first convert the properties and then call