Make DP3 buildable without Armadillo

Created by: aroffringa

CMake disables DDECal when armadillo is not found, but this results in undefined references:

./CMakeFiles/DPPP_OBJ.dir/DPPP/DPRun.cc.o: In function DP3::DPPP::DPRun::makeSteps(DP3::ParameterSet const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, DP3::DPPP::DPInput*)': DPRun.cc:(.text+0x3228): undefined reference to DP3::DPPP::DDECal::DDECal(DP3::DPPP::DPInput*, DP3::ParameterSet const&, std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&)' collect2: error: ld returned 1 exit status DPPP/CMakeFiles/DPPP.dir/build.make:343: recipe for target 'DPPP/DPPP' failed make[2]: *** [DPPP/DPPP] Error 1 CMakeFiles/Makefile2:313: recipe for target 'DPPP/CMakeFiles/DPPP.dir/all' failed make[1]: *** [DPPP/CMakeFiles/DPPP.dir/all] Error 2 Makefile:129: recipe for target 'all' failed make: *** [all] Error 2

Since only the ionospheric screen fitting depends on armadillo, it's probably best to only skip building that instead of DDECal entirely.