Add Custom Unit for Socket subtype

This small improvement for usability to use custom unit suffix beyond current build-in Socket subtype.

• when select "Custom Unit" text box will appear for type custom unit text after number ,You will don't need to type unit name after socket name for better readability.

This proprosal to allow user create custom simulation input units for varies simulation fields for science and engineering.

Example for Electronics Simulation

• Voltage (V)
• Current (A)
• Resistance (Ohm)
• Capacitance (F)
• Inductance (H)

etc.