Modify solid-to-solid contact BC and add new test

[danielwolff1]

Outset

When trying to run a 2D contact simulation I realized that there is currently no BC enum for this case implemented.

What was changed?

In this PR, I implemented a potential solution, suggesting to rename the existing enum solid_to_solid_surface_contact simply to solid_to_solid contact, since we're anyway checking for the dimensionality of the geometric entities in contact before adding the BC string to the 4C input file.

Was the new implementation tested?

I adapted the unit test for surface-to-surface contact accordingly and added a new unit-test for curve-to-curve contact. All unit tests passed on my local machine.

Sidequests

I noticed that parts of the README.md where outdated / not accurate, so I adapted them accordingly as well :)

[isteinbrecher]

Thanks for the contribution @danielwolff1 , I only have some minor comments.

@knarfnitram this will break the "old" approach of defining surface-to-surface contact, I think you might be the only one using that. If you need it we can keep the solid_to_solid_surface_contact enum as a legacy enum.