Clean up and refactoring related routing method indices

Erik (@ekluzek) suggested this, and use [write_restart_pio.f90](https://github.com/ESCOMP/mizuRoute/pull/482/files/2c01d88e1cf750815d8a3ac2aa3e94a61cdfe57b#diff-fc17794cea08939ea2153c09862e61f01e2dd7cf4536bf3f03070e580f98290f) etc.

``` fortran
pure integer function get_routing_index(routing_method_name) result(route_indices)
implicit none
integer(i4b) :: route_indices(3) ! route_indices(1)->routeRunoff index, route_indices(2)->volume index, route_indices(2)->inflow index
select case(routing_method_name)
case(accumRunoff)
route_indices(:) = -1
case(kinematicWaveTracking)
route_indices(1) = ixRFLX%KWTroutedRunoff
route_indices(2) = ixRFLX%KWTvolume
route_indices(3) = ixRFLX%KWTinflow
case(ImpulseResponseFunction)
route_indices(1) = ixRFLX%IRFroutedRunoff
route_indices(2) = ixRFLX%IRFvolume
route_indices(3) = ixRFLX%IRFinflow
case (...)
.....
default
route_indices(:) = -999
end select
end function get_routing_index
```

With this function, may allow us to use do loop to write output etc.