Specifying reservoir locations as GRID_CELL_INDEX (rather than using ID) is confusing

[thurber]

Specifying reservoir locations as GRID_CELL_INDEX (rather than using ID) is confusing and counterintuitive for users. We should consider updating the expected input file format to always use and respect grid cell IDs, rather than trying to recast the internal ID as an arbitrary zero-based index. We currently handle this properly for the domain file, but not for the reservoir file (therefore making it even more confusing).