Instantiation of cells

[Rishabhgoyal07]

@heitzmann, I have extracted a cell from a library, how can multiple instances be created of that cell, so that whenever we need that cell, we can just place the instance of it.

[heitzmann]

Please, read the documentation: https://heitzmann.github.io/gdstk/
The Getting Started section mentions it in the 3rd sub section.