Add support for CppAD

[artivis]

Add support for the CppAD library

CppAD A C++ Algorithmic Differentiation Package (web - github).

This PR adds support for the CppAD::AD type (both forward & reverse).

Find a doc page here (mostly copied from the Ceres/Jet doc page).

The Jacobians produced by CppAD are compared against manif analytics ones in unit tests.

Marking this as draft until I find a better way to handle the conditional operators as the current ones introduce some unnecessary evaluation.

Closes #133.

[codecov[bot]]

Codecov Report

Merging #238 (3db9697) into devel (ab560a3) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            devel     #238   +/-   ##
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  Coverage   98.17%   98.17%           
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  Files          55       56    +1     
  Lines        1754     1757    +3     
=======================================
+ Hits         1722     1725    +3     
  Misses         32       32