Add local and body frame transforms

[ryanpavlick]

Summary

• add SEZ and NWU local frame conversion modules built on the existing ENU/ECEF math
• expose explicit ECEF<->local rotation matrices for ENU, NED, SEZ, and NWU
• add an aerospace body-frame layer with conversions to and from NED, ENU, SEZ, NWU, ECEF, and geodetic coordinates
• document the new frame conventions and add regression coverage for frame conventions, round trips, and matrix behavior

Testing

• python -m pytest src/pymap3d/tests/test_body.py src/pymap3d/tests/test_local_frames.py src/pymap3d/tests/test_enu.py src/pymap3d/tests/test_ned.py src/pymap3d/tests/test_aer.py -q
• python -m pytest src/pymap3d/tests/test_geodetic.py src/pymap3d/tests/test_spherical.py -q