Feature/cmake targets

[pascalau]

This PR allows kindr to be built completely independently, with catkin or colcon without brittle if/else statements. It exports the header library as the target kindr::kindr in a modern cmake fashion.
Example usage for downstream consumers:

find_package(kindr REQUIRED)
add_library(consumer_lib ...)
target_link_libraries(consumer_lib kindr::kindr)

Tedious working with variables ${kindr_*} and worrying about include directories becomes obsolete.
If downstream targets also export dependencies correctly this simplifies dependency management.

Usage with different build systems:

# pure cmake
mkdir build && cd build
cmake .. -DBUILD_TESTING=ON
make
make test
# colcon assuming kindr cloned in src
colcon build --cmake-args -DBUILD_TESTING=ON
colcon test
# analog for catkin tools
catkin build --cmake-args -DBUILD_TESTING=ON
catkin test

Originally filed a year ago against the ASL fork ethz-asl#1. Therefore it might needs some cleanup.