Support Retrieving Well Vectors from {I,X}{WEL,CON}

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.

tests/FIRST_SIM_THPRES.DATA

@@ -13,10 +13,18 @@ UNIFIN
 DIMENS
  10 10 10 /
 
+WELLDIMS
+-- Item 1: NWMAX  (Maximum number of wells in model)
+-- Item 2: NCWMAX (Maximum number of connections per well)
+-- Item 3: NGMAX  (Maximum number of groups in model--excluding FIELD)
+-- Item 4: NWGMAX (Maximum number of wells or child groups per group)
+-- NWMAX  NCWMAX  NGMAX  NWGMAX
+   6      3       1      6
+/
+
 EQLDIMS
    10 /
 
-
 GRID
 DXV
 10*0.25 /