Improved stage nesting

• Improvement: Add BufferWriter::{process, read}
• Improvement / Change: Magnetron::{process, process_nested} is now a combination Magnetron::{prepare, prepare_nested} and BufferWriter::process
• Improvement / Change: Homogenize BufferWriter::read_x_write_y
  • Add out_level / out_levels argument in every function
  • out_level / out_levels receives an Option<f64> defaulting to an amplitude of 1.0 if absent
• Improvement: Add StageActivity::Observer