Implement variables for shutoff valves

[tasseff]

Refer to #28 for a discussion on shutoff valve data structures. Obviously, the variables that already exist for pipe objects will remain for pipes with shutoff valves. However, each shutoff valve will require the addition of a binary variable, wm.var[:nw][n][:x_so][a], which defines if the shutoff valve within pipe a has been opened (1) or closed (0) by the operator at time n.

[tasseff]

Actually, I will propose naming the binary variables for shutoff valves wm.var[:nw][n][:x_open][a].

[tasseff]

Note that naming conventions for component status variables have changed. Now, :z_shutoff_valve indicates whether a shutoff valve is open (1) or closed (0).