Use Matrix3.storage directly instead of _m3storage

[spydon]

This PR:

• Does the same as Use Matrix2.storage directly instead of _m2storage #286 but for Matrix3
• Removes all factory constructors (except Matrix2.rotation)
• Simplifies many calculations

Since this is based on the branch for #286 that should be merged first and then this branch should be updated.

[coveralls]

Coverage: 49.011% (-0.5%) from 49.501% when pulling 7a1d48c on spydon:fix/matrix3 into 1e4d000 on google:master.