Add interpolation and static properties to Param Desc and its corresponding interfaces.
Add macros to import and export interpolation and to export static
Turn z_depth into a ValueBase type
Layer: Add functions to set and get param interpolation and to set interpolation defaults. As example I set default interpolation for z_depth to be CONSTANT. To be fixed later.
Circle: add members to set / get parameter interpolation and add a call to set interpolation defaults at constructor. Set default interpolation for radius as CONSTANT as example. To be redefined later.