Different types of nodes #8

Source node(s) - Outputs only

• Flat value: value (float)+ label. Corresponds to the pathsim.Constant block
• Pulse: amplitude, pulse_duration, t_rise, t_fall, tau, duty (all floats). Corresponds to the pathsim.PulseSource block

Other nodes:

• Process node - Standard node with i/o as with attributes: residence time, source and initial condition
• Scope node - Inputs only - will be used to define which processes are fed into the scope for output/plotting

We need:

• Replace the Add Node button by a draggable menu like the drag and drop tutorial from which users can choose between different types of nodes
• write the correct type to json (right now they are all "custom")