[ENHANCE] Extend kMedium to handle elastic wave propagation

[djps]

Description of the Enhancement

Extend kMedium wto handle elastic wave propagation, which it currently can not do.

Justification for the Enhancement

This is necessary to perform elastic wave simulations.

Impact of Changes

As an extension of an existing class it should not break anything.

Additional Context

There is both a manditory sound_speed as well as two optional sound speeds: sound_speed_compression and sound_speed_shear. Perhaps it is worth overwriting the compression option with the default?