Differentiable PTO impedance

[cmichelenstrofer]

Description

The PTO impedance was not differentiable. This was OK because the impedance is just created once during the solve, and does not depend on x_wec or x_opt which is what we differentiate w.r.t. But in the EWTEC LUPA study we want to get gradients w.r.t. PTO parameters.

Type of PR

• Bug fix
• New feature
• Documentation
• Other: (specify)

Checklist for PR

• Authors read the contribution guidelines
• The pull request is from an issue branch (not main) on your fork, to the main branch in WecOptTool.
• The authors have given the admins edit access
• All changes adhere to the style guide including PEP8, Docstrings, and Type Hints.
• Modified the documentation if applicable
• Modified or added a new test
• All tests pass
• Reference or close any relevant issues

Additional details

This is needed for the LUPA EWTEC study. Other parts of the PTO and code are likely still not differentiable.

[cmichelenstrofer]

the main issues were:

1. indexing in an assignment (we already knew autograd doesn't like it)
2. using np.block function (discovered autograd doesn't like it)