Bugfix/time compute

modify the code to romove unnecessary 'if'

refactor increment function and fix the way of time calculation
add GenerateTimeItems function for time generate to replace GenerateIntItems function
modify the parse process of step

feat: time step format extend

The time step format has been expanded from only supporting second(s)
to supporting year(Y),month(M),day(D),week(w),hour(h),minute(m),second(s)

fix:fix path of zd in GetExeDir method

运行"go run src/zd.go -c demo/t.yaml"出现"... is not a vaild ZenData dir."错误，由config.go文件的InitConfig方法爆出，
是因为在mac平台上，file.go文件的路径是写死的，注释之后就正常了。