[check_swatinit] More robust SWL/FILLEPS handling

Currently, if SWL is missing in INIT file, it is filled with zero, and a warning on FILLEPS is emitted.

Change to:
* If no SWL in INIT file and no SWL keyword in deck: Use SWL from SWOF/SWFN tables
* If no SWL in INIT file and SWL keyword in deck: Error hard and ask for FILLEPS (/or/ fill in SWL from deck ?)

Check if SWU should be handled identically.