[Bug]: `simulation.clear_cache()` crashes if simulation contains line DEVC devices

[wk9874]

What happened?

I've loaded a set of results using fdsreader.Simulation(), and then after parsing some data from it, I use simulation.clear_cache() to reduce memory usage. However for examples which contain a line DEVC device, eg

&DEVC XB=1.5,1.5,0,0,0,2.9, QUANTITY='VISIBILITY', ID='visibility_line', POINTS=100, STATISTICS_START=0., STATISTICS_END=5. /

clear_cache() crashes with this error:

    def clear_cache(self):
        """Remove all data from the internal cache that has been loaded so far to free memory.
        """
        for element in self._elements:
>           element.clear_cache()
E           AttributeError: 'list' object has no attribute 'clear_cache'

When inspecting the simulation object, it looks like this is because the line DEVC device has been loaded a list of Device() objects:

print(sim.devices._elements[6])
[
    Device(id='visibility_line', xyz=(1.5, 0.001, 0.001), quantity=Quantity('VISIBILITY')), 
    Device(id='visibility_line', xyz=(1.5, 0.001, 0.02929), quantity=Quantity('VISIBILITY')),
    ...
]

Steps to reproduce

• Run an FDS case which includes a line DEVC device definition, eg the one mentioned above
• Load the simulation with simulation = Simulation(results_dir)
• Try to clear the cache with simulation.clear_cache()

FDS version used

6.9.1

fdsreader version used

1.11.7