[autotools] Move HostGridAccessor to own header hostgridaccessor.hh

Having HostGridAccessor in a separate files makes it possible to resolve
dependency issues that left headercheck very unhappy.
[autotools] Fix various Makefile.am files.
[Test] Small improvements in /tests.
[Cleanup] Fix headercheck for subdomaingrid
[CMake] Initial CMake support

Tests are not automatically built, because the folder is named tests
instead of test.
Boost is not checked, maybe this has to be done, too.
[Tests] Rename tests/ to test/ for consistency and to make CMake happy

Thanks to Christoph for pointing out the issue.