[ENHANCE] Extend kSource to handle elastic wave propagation

[djps]

Description of the Enhancement

Extend kSource to handle elastic wave propagation, which it currently can't do.

This entails extending the source types to be either particle velocity or stress terms.

Justification for the Enhancement

This is necessary to perform elastic wave simulations. It also implements blocks of code which are unimplemented.

Additional Context

Needs testing.