strict mode oasis should write the S_CELL_OFFSET into the CELLNAME table

[stefanottili]

The klayout/strm2oas default is to write strict mode oasis, but the CELLNAME table currently doesn't contain the S_CELL_OFFSET property.

In order be able to take advantage of strict mode and to allow multi threaded oasis readers random access, the CELLNAME table has to contain the S_CELL_OFFSET property.

Currently one has to use --write-std-properties=2, but that shouldn't be necessary to write S_CELL_OFFSET in strict mode.

Leave that switch for S_BOUNDING_BOX, which tools usually compute on their own anyways.