Depth-weighting to average across select 3D model layers

[lawrenceabird]

extract_field_layer() extracts data from a single model layer. In order to average across multiple layers, layers must be "weighted" based on the layer thickness.

See $ISSM_DIR/src/m/extrusion/DepthAverage.py for starting point.

This functionality should be imbedded into:

• extract_field_layer() as (for example) extract_field_layer(md, field, layer = [1, 8])

Also affects:

• plot_model_field() - update to accept list of layers