Issue 945 - Updates the STF and force implementations to match SPECFEM2D Fortran

[lsawade]

Description

The changes should reflect SPECFEM2D's PR #1259 that updates the implementation of the source time function to more accurately match SPECFEM3D.

Problem

The way the acoustic force source_array was applied in the medium/compute_source had the wrong sign as a result of legacy code. One result of this is the definition of the source angle was inverted.

Solutions

• Removing of negative signs in the compute_source_time_function member functions of the Ricker, Delta, dGaussian functions, e.g.
• Updating sign and cosines in the angle computation of the elastic force
• change sign of the operation of the potential in medium/compute_source<acoustic> from
  acc(...) += source_array(...) to acc(...) -= source_array(...)

Issue Number

Closes #945

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• THE DOCUMENTATION BUILDS WITHOUT WARNINGS/ERRORS
• I have added labels to the PR (see right hand side of the PR page)
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms

[lsawade]

@danielpeter the tests here should fail right now since i didn't update the SH tests, which should fail due to the fixes. But maybe you could take a quick look just to make sure I'm getting "everything".

SPECFEM2D's PR #1259

[Rohit-Kakodkar]

Can you add a brief discription of what was wrong with original implementation as a comment here (just a few lines) ? Just as a reference to point to incase some one has question.