Rename axial_stiffness and axial_damping

[ufechner7]

The unit of axial_stiffness is N/m. But we use the definition now:

 axial_stiffness: 614600.0     # unit spring constant coefficient [N]

which is wrong.

See also: #93

[ufechner7]

My current suggestion: Use the terms:

• unit_stiffness and
• unit_damping

Then the stiffness and the damping of a tether segment would be:

longitudinal_stiffness = unit_stiffness/segment_length
longitudinal_damping = unit_damping/segment_length

[rschmehl]

Instead of "axial" you might want to use "longitudinal". These terms are pretty common: "longitudinal tether dynamics" vs "lateral tether dynamics". The first one is about the traveling of signals along the tether, while the second is about the sagging and straightening behavior (Fourier modes).