Fix error attribute index for output reader

[wqycg]

The attribute index starts from 1 rather than 0.

typedef enum {
    ENR_demand      = 1,
    ENR_head        = 2,
    ENR_pressure    = 3,
    ENR_quality     = 4
} ENR_NodeAttribute;

typedef enum {
    ENR_flow        = 1,
    ENR_velocity    = 2,
    ENR_headloss    = 3,
    ENR_avgQuality  = 4,
    ENR_status      = 5,
    ENR_setting     = 6,
    ENR_rxRate      = 7,
    ENR_frctnFctr   = 8
} ENR_LinkAttribute;