materials are owned by each layer
add ability to change opacity for each layer with dat-gui panel
add checkbox to toggle visibility for dem sides and bottom
hide clip option when layer type is point
clean up DEMPropertiesWidget layout

- remove spacers that causes blank space when the group of widgets is hidden.
- remove transparency option for sides and bottom. opacity depends on that of main dem object.
use 0.6 as default value instead of 0.5
altitude mode: relative to DEM, DEM + attr value, absolute value and attr value (and z value)

- error handling in converting value to float
- rename HeightWidgetFunc.RELATIVE_TO_Z to Z_VALUE
- clean up StyleWidget class