Bugfix: Fix bad initialization for time steps > 0

[sanathkeshav]

Fixes bad initialization of nodal degrees of freedom for time steps > 0, which was causing a slight performance hit.
Introduces a linear extrapolation, which is a better initial guess and reduces the iteration counts a bit.
No changes to the results themselves.
Option for the user to specify the reference stiffness in the input JSON used to construct the fundamental solution.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.