BFI target function

Added BFI-target function to mHM:

• opti_function=34:

  !> (34) SO: Q: (1 + |BFI_o - BFI_s|)(1 - KGE)
  !>             BFI_s = mean_t(<q_2>) / mean_t(<q_total>)
  !>             BFI_o = given in separate namelist per domain
  !>             <.> is a spatial average

• you can either specify a baseflow index per domain or you can let calculate this by mhm intern with the Eckhardt filter
• a new namelist was added to the mhm.nml:

  !******************************************************************************************
  ! SETTINGS FOR OPTIMIZATION for baseflow-index (opti_function = 34)
  !******************************************************************************************
  &BFI_inputs
  !> Calculate BFI from discharge time series with the Eckhardt filter
  !> Eckhardt et al. (2008, doi: 10.1016/j.jhydrol.2008.01.005)
  !> This option **requires** one gauge per domain at the outlet of the basin.
  BFI_calc = .true.
  !> baseflow index per domain. Only needed if not calculated (BFI_calc = .false.)
  !> You can overwrite single BFI values to not calculate them internally (if BFI_calc = .true.).
  ! BFI_obs(1) = 0.124
  ! BFI_obs(2) = 0.256
  /

• when calculating internally, you need to have exactly one gauge per domain