Char speed control: return Strahlkorper of excision surface

[geoffrey4444]

Task list:

• [x] Add Jacobians & mesh velocities to interpolation interface
• [x] Compute inverse spatial metric (for normalizing the normal vector), shift and mesh velocity in contracted with the spatial Jacobian.
• [x] Add compute tag for char speeds on excision Strahlkorper. This needs inertial lapse, shift in grid frame, mesh velocity in grid frame, and constraint gamma1 to be interpolated. We then compute `-lapse \pm (shift^i{grid} + v^i_{grid})n_i^{grid}`
• [x] Observe char speeds in SingleBH executable
• [x] Add ExcisionBoundaryA/B to BBH executable (can hand off to Mark/Kyle if/as necessary)
• [x] Observe char speeds on excision boundary in BBH (can hand off to Mark/Kyle if/as necessary)

Note: Sending to the control system will be implemented by @markscheel and @knelli2

[geoffrey4444]

@gsb76 will add this to the BBH executable (straightforward), and compare spec vs spectre char speeds for half an orbit or so