Templated local parametrization for ceres

[DmitriyKorchemkin]

• Added templated local parameterization class
• Slightly adjusted RxSO2::cast / (Rx)SO3::operator* in order to be compatible with ceres::Jet

In order to work around a corner-case of SO2::Tangent being Scalar (and not Vector<Scalar, 1>), helper trait is_mappable_type_v is introduced (using completeness of Eigen::internal::traits<T>, since just having ::Scalar type is not enough, since ceres::Jet has this type defined, but needs to be considered a scalar). This type trait is only needed for group-agnostic code and end-users do not need to use it.

Added also a simple Lie-group averaging example (on a complete graph in order to simplify test data generation).

[coveralls]

Coverage increased (+0.03%) to 99.013% when pulling 7366e2d on DmitriyKorchemkin:ceres-local-parameterization-2 into e10eb6e on strasdat:master.

[DmitriyKorchemkin]

@sergiud , are there any problems left?

I have a ceres::Manifold-based implementation ready; it reuses test-suite and type traits.
Should I add it to current pull-request, or wait till it gets merged in and open a new one?

Thanks

[sergiud]

Your PR looks good to me. @strasdat needs to review it.

[NikolausDemmel]

I have a ceres::Manifold-based implementation ready; it reuses test-suite and type traits.
Should I add it to current pull-request, or wait till it gets merged in and open a new one?

IMHO a second PR would be best (but I don't have any authority here). You could already open it, if you want, and note that it depends on this PR. Thanks for your contribution! This will be very useful.