Install header files and support older versions of Eigen

[jmirabel]

Opening a PR rather than an issue as the changes are minimal.

Would you consider the two following changes:

• install the headers. I'm not sure how people work with this library but following the source install instructions gives an environment with an .so file and no header file. This makes the lib unusable in C++ in downstream package, unless one use it in a catkin devel environment.
• Using Eigen::Matrix instead of Eigen::Vector makes the lib support older version of Eigen. In this PR, I changed the minimal version to 3.3.0 but I'm sure it would work with earlier version as well. This helps when using the lib on old distrib.

[pantor]

Thanks for your input!

The header issue has been fixed in another PR, and we now support Eigen from 3.3.7 onwards. I hope this helps!