Error in pressure_drop dump when using mixed_convection solver

[Nino-ndrk]

By enabling pressure_drop in Dump section to get information of the detailed pressure drop data to CSV file, I get an error when mixed_convection is also enabled.
The error seems related to a formatting issue of the pressure drop calculation coming from MixedRegion.
The error traceback is following reported:

  File "path_to_Python_installation\site-packages\dassh\reactor.py", line 1147, in axial_step
    self._calculate_asm_temperatures(self.assemblies[ai], ai,
  File "path_to_Python_installation\site-packages\dassh\reactor.py", line 1248, in _calculate_asm_temperatures
    asm.write(self._options['dump']['files'], gap_temp)
  File "path_to_Python_installation\site-packages\dassh\assembly.py", line 760, in write
    for k in reg._pressure_drop.keys():
AttributeError: 'numpy.float64' object has no attribute 'keys'

[Nino-ndrk]

The issue arises from replacing self._pressure_drop attribute of RoddedRegion class which was a dictionary defined as:

self._pressure_drop = {
            'friction': 0.0,
            'spacer_grid': 0.0,
            'gravity': 0.0}

with a float value inside MixedRegion class. It is suggested to set a new attribute as self._pressure_drop_tot that will contain the new mixed convection solver results and keep the old dictionary that can be filled after the calculation of _solve_system method. In such a way it would be possible to give again in the main summary report also the different contributions of DP (friction, gravity, spacer) along the bundle.