add mdot , ndot etc as attributes to stream objects rather than massflow objects

[sivarammurugan]

At present , the user is required to explicitly create a mass flow object and attach to the stream name , before accessing mdot , ndot etc. For example ,

overhead = f.stream()
massflow_overhead= f.massflow(sn = overhead)
massflow_overhead.mdot = 100

However , it would be more intuitive , if mdot and ndot etc are accessible as attributes directly under the stream object as below

overhead = f.stream()
overhead.mdot = 100