This commit makes class 'Opm::EclIO::ERst' able to open separate
restart files (*.X000n, *.F000n). Specifically, we refactor the
existing class constructor body into a new helper function
'initUnified'. We add a new helper function 'initSeparate' that
builds the requisite indices in the case of a single report step,
and make the constructor body call 'initUnified' or 'initSeparate'
depending on whether or not the SEQNUM keyword exists in the restart
stream.
Add two new unit tests to exercise the new ability.
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.
