Roughness as a parameter of the pipe rather than the slurry

[rcriii42]

Currently the pipe roughness (epsilon) is a parameter of the Slurry Object:

class Slurry():
    def __init__(self, Dp=0.762, D50=1.0/1000., fluid='salt', Cv=0.175, max_index=100):
        self._max_index = max_index
        self._Dp = Dp
        self._epsilon = DHLLDV_constants.steel_roughness

    ...
    ...

    @property
    def epsilon(self):
        return self._epsilon

    @epsilon.setter
    def epsilon(self, e):
        self.curves_dirty = True
        self._epsilon = e

As a result, you can set the roughness for an entire pipeline (pl=Pipeline(...); pl.slurry.epsilon = new_roughness), but not for individual Pipes in the pipeline. Making roughness a parameter of the Pipe object, and getting the Pipeline to respect that, will allow this level of detail.