This commit extends function RestartIO::load() to retrieve most of
the well restart data (aggregate flow rates per well connection,
or per well) from the standard ECL restart vectors IWEL, XWEL, ICON,
and XCON. We still prefer the dedicated OPM_* variants if
available, but this commit brings us closer to being able to retire
OPM_XWEL.
While here, also add WELLDIMS to FIRST_SIM_THPRES to restore the
STORE_THPRES unit test.
