Rename docu_dir -> doxy_dir to reflect its purpose
Only copy Doxygen stylesheet if it actually exists
Consider directory itself to be part of datafiles

Previously the directory name was just entered as part of a glob; the
new version uses the satellite name as a directory and also makes sure
that this directory is available to run tests in, even if there are no
datafiles (the directory itself is now considered a datafile)
Correct mixup between option and cache variables

Options are boolean flags that are stored... in the cache, whereas
cache variables are also stored in the cache, but can have a variety
of types (including booleans). It was possible to confuse the two,
because if you set it as a variable on the command line, it will
override both.
Allow more variety in unit test program file names

They can now be called test_*.cpp or *_test.cpp
Specify source directory at the highest level

Some projects also have side directories for utilities etc.
Make project file easier to customize

Note where opm-core specific parts are, and put dependency listing to
the top where it is more noticable.