Fix maybe-uninitialized warnings

[papjuli]

Fix some code that result in maybe-uninitialized compiler warnings:

• best_model in ransac and loransac can be uninitialized -> use optional,
• initialize Eigen matrices to 0 in EstimateRigid3d and other similar functions.

Also, don't turn the warning type off CMakeLists.txt.

Unfortunately, there are 3 other such warnings that originate outside, namely in boost graph.

[ahojnnes]

Thank you. LGTM.