This makes the facility usable for the restart read/write code.
Specifically, import the I/O classes into namespace Opm::ecl, and
place the files in physical location opm/io/eclipse, and move the
test utilities to new top-level directory 'test_util/'. While here,
discontinue the 'testutil' static library since most of its features
are now available in the main 'opmcommon' library. This does entail
compiling a few of the test_util/ CPP files multiple times, and
adding the objects to each executable independently.
