Enum for layer management

[dolphingarlic]

KLayout supports layer property (.lyp) files we would like to use for better layer visualization/management. However, this would require standardizing our layer conventions (e.g., the layer numbers for different types of layers). As a first step, we should create Python enums to keep track of these conventions.

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Layer assignment (up for debate)
0 - low-current ebeam (pos-tone)
1 low-current ebeam (neg-tone)
10 - high-current ebeam (pos-tone)
11 high-current ebeam (neg-tone)
100 - photolitho (pos-tone)
101 photolitho (neg-tone)

Resistor layers?

Via layer, resistor contact layer?

[reed-foster]

A couple helpful starting points
https://github.com/qnngroup/reedf-layouts/blob/main/layers.py
https://github.com/qnngroup/reedf-layouts/blob/main/visualize_stackup.py

Some relevant blog posts:
https://qnn.mit.edu/wordpress/archives/30781
https://qnn.mit.edu/wordpress/archives/30814