SubDyn: add rectangular beam properties to JSON output

[RBergua]

This PR is ready to be merged.

Feature or improvement description
SubDyn allows to model rectangular beam elements. Unfortunately, it’s not possible to visualize them with the 3D visualization tool from Emmanuel Branlard (https://ebranlard.github.io/viz3Danim/)

Currently, when rectangular beams are modeled in SubDyn (see for example: https://github.com/OpenFAST/r-test/blob/main/modules/subdyn/SD_CantileverBeam_Rectangular/SD_CantileverBeam_Rectangular.dat), and the user requests the JSON output files (i.e., OutCBModes = 1 and/or OutFEMModes = 1), SubDyn does not recognize the rectangular element and writes:

Example of current visualization:

To visualize rectangular elements, it is necessary to update the “shape” value and add new keys like “SideA”, “SideB”, and “SideA_dir” instead of “Diam”. For reference, the “SideA_dir” key corresponds to a vector used to orientate the SideA of the rectangle (local beam x_e axis). In this case, we work with the “SideA_dir” instead of the spin angle (MSpin) for simplicity. This “SideA_dir” is useful for SubDyn and HydroDyn as both modules use the same convention.

Example of new output (SD_CantileverBeam_Rectangular.SD.CBmodes.json):

The current modification writes the necessary outputs and it does not break the current parser of viz3Danim.

Another PR has been opened at the viz3Danim side (ebranlard/viz3Danim#1) to visualize the rectangular elements.