This commit extends the EclOutput constructor to support letting the
client specify a particular open mode. This in turn prepares using
EclOutput as the basis for a restart file writer. Passing
mode=openmode::out is the common case for a new restart file, while
mode=openmode::app is the common case for an existing restart file.
Specifically, add a member function
EclOutput::message(const std::string&)
that outputs the string as an 'eclArrType::MESS'. This is in order
to support outputting 'STARTSOL' and 'ENDSOL' demarcations in a
restart file.
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.
