(mostly) velocities in *grid.py - rigid body velocity (linear and angular) contributions of strip/non-lifting surfaces

[kccwing]

I guess this bit is quite related to the original generate_zeta_timestep_info in aerogrid.py - in there I realised the rigid body velocity (linear and angular) for the most general expression for the strip velocities were missing (in eq 54 in screenshot attached).

Specifically here, it seems that in line 92, matrix_nodes used was already inclusive of the rigid body position -> effectively it's the strip coordinates in G frame already (as processed in line 85) - but then we premultiply with another Cga afterwards in line 96 - possible typo?

Originally posted by @kccwing in #249 (comment)

creating this to document future development/verification effort required - two sources of rigid body (translational and rotational) velocities in nonliftinggrid/aerogrid.py, and also in unsteady/unsteady_utils.h (compute_resultant_grid_velocity) - how to reconcile the differences? which velocity is included where?