Controlling the way a layout reader integrates cells into an existing layout

[klayoutmatthias]

The "poor man's layout merge" is implemented by reading two layout files into the same object.

By doing so, existing cells will be extended by the content of the new file.

However, this scheme may not be desirable always. Other schemes are possible.

In this context, I received a user request with the following options to control this feature:

• The current behavior.
• Always overwriting, in case of conflict, the cell already in the layout.
• Never overwriting, in case of conflict, the cell already in the layout..
• Automatically renaming, in case of conflict, the cell to be read.

If the existing cell is a ghost cell, the new cells should always be integrated (current behavior).

NOTE: For ghost cells in the new layout, the current behavior can't be implemented always as during sequential reading, the nature of a cell (ghost/normal cell) cannot be determined.

This applies to reading GDS and OASIS files mainly.

[klayoutmatthias]

Issue has been addressed with the issue_666 branch. It has been merged to master with PR #677