Feature/cmake

[markusgft]

• using pure cmake to build all the projects
• minor fixes regarding target linking and explicit template prespec
• added FindIpopt.cmake

[markusgft]

• properly resolve lapack dependency (don't put in package.xml since it's optional)

• use add_library(... INTERFACE) to create interface libraries for header-only parts.

• make sure documentation is up to date and all examples get linked correctly

• remove remaining clang-format and clang-tidy commands, remove clang compiler scripts, add to documentation how to change the global settings for compilers

• test exporting solver flags

• experiment with target_compile_definitions

• make sure doc does not get installed without explicit command to do so

[markusgft]

@jcarius the cmake-only version should be fit for testing - I would be curious to see if it works out of the box with your local CT-based developments (it should!).
It still supports catkin, i.e. catkin build ct_optcon -DBUILD_EXAMPLES results in the same behaviour as before.
Important note: you may need to adjust your downstream projects in one single aspect. That is ct cannot be listed as catkin_dependency anymore. Now it needs to go under the standard DEPENDS tag.