Robust way to deal with UseLatex erros.

Rationale:
UseLatex just send error on missing tools, we have to make it robust
to prevvent failing of main CMake run.

- make doc/reference_manual/CMakeLists.txt independent CMake project
- call it from main CMakeLists.txt through execute_process
- omit target ref-doc in doc/reference_maual
  call make pdf form main makfeile, tergar ref-doc (keep things simple)
Artificial target unit_tests/gtest_mpi_obj.

Seems that when unit tests are called from Jenkins in parallel,
it fails due to recompilation of common sources.

- artificial target target builds gtest and our gtest_mpi.cc and
possibly other files common to more unit test and hopefully fix it.
Jenkins YAML: fixes and new naming scheme.

- new scheme for display names: PLATFORM BUILD-TYPE / something / something
  .. changes in real package names will follow
- fix clean of workspace
- add build libraries before running unit-tests
- try to limmit number of parallel jobs to avoid OUT OF MEMORY