Moment source for elastic wave

[SouzaEM]

The strange oscillations of the current implementation of the moment source must be fixed and a more intuitive dictionary parameter should be created for the selection of moment sources.

[SouzaEM]

The elastic wave propagator will be verified with analytical solutions from the book "Quantitative Seismology - Second Edition" from Aki and Richards. The body force source will be verified against Eq. (4.23), and the moment tensor source will be verified against Eq. (4.32).

[Olender]

UNfortunatelly, this is now part of the temporarily private repo. However, it will become public in a year. I will close this issue for now, since its solved elsewhere