HOWTO rivers

! In riverinfo.dat it is now allowed to have:
!   empty lines
!   comments using # or ! - everything to the right of the comment character
!   is discarded
! format of this file:
! a line with an integer number specifying the number of rivers to read
! lines with the following information for each of the rivers:
! i  j  name <zl zu>
! i,j specify the global indices of the position of the river
! name (without spaces) is used as key to the NetCDF file with the actual data
! zl and zu specifies the depth range over which the river influences the
! water column - zl and zu are depths i.e. measured from the surface. Negative
! values have the following meaning - if zl < 0 use bottom and if zu < 0 use
! surface. zl and zu are optional but either none or both must be specified
! number of rivers