Better support tests in out-of-tree build.

Add path to data directory by preprocessor definition so that
unit tests can be run from any directory.

Merge Issue 18: Cleanup of includes

(yes, I'm one of 'those' programmers)

Use "" for local includes
Use <> for includes from include directories

Add all directories with files to include to CMakeLists.txt
Make all linux folder based includes global (remove the folder structure)

Use externalproject_add to manage mod->hpp depenedencies.

Builds modcc from modparser as an external project;
executable will reside under BUILD/external/bin where
BUILD is the CMake build directory.

CMake hackery (see 'build_all_mods' target) should generate
required module .hpp files from .mod files before the
cellalgo library is built, if the header files are
missing or out of date.

This should also suffice for performing out-of-source
builds.

Cell construction

Read cell from swc file