cmake: add UseMultiArch cmake module

Source: https://raw.github.com/OPM/opm-upscaling/master/cmake/Modules/UseMultiArch.cmake

The cmake module fixes wrong multiarch installation paths when cmake is
called from debhelper
cmake: build documentation by default

if doxygen is present, build the documentation in the all target
cmake: install documentation to share/doc/rtrlib/
cmake: move rtrclient install statement to tools/CMakeLists.txt

All cmake statement related to files in tools should be in
tools/CmakeLists.txt
This commit moves the cmake install install statement for the rtrclient
tools to tools/CmakeLists.txt.
debian: add debian packaging files