Make conversion function between frames

[mceowen]

Mikipilot, optgui, and SkyeFly should all have matrices to convert between frames consistently

Currently:

• SkyeFly is [Z_down, X_N, Y_E]
• Mikipilot is [X_N, Y_E, Z_D]
• optgui is [pos.x pointing left, pos.y pointing down] = [r[2], -r[1]] in SkyeFly (for reference)