Replace all remaining file_name with filename

[teutoburg]

I guess those were missed in #147? Anyway, they caused some errors and also a hard-to-trackdown weirdness of wrong detector layout (imaging layout was used in IFU mode). This fixed it.

[hugobuddel]

These file_names should not cause any problems. In fact, we changed some of these before (not sure whether in #147 or later), but we had to revert them (to get the IFU mode working).

The __init__() of effects should not have an file_name parameter, and therefor the kwargs in the yaml files should never have an file_name keyword them.

However, these file_names only appear in the properties, and thus only in the values of the kwargs in the yaml files, not the keys.

Now arguably, we can also change these to filename, and I would be fine with that. However, we should first make sure that we understand what we are doing, because these are self-consistent and should not have caused any "hard-to-trackdown weirdness".

In particular, the current ScopeSim code will be broken if we make this change now:

scopesim/effects/metis_lms_trace_list.py
310:        filename_det_layout = from_currsys("!DET.layout.file_name", cmds=self.cmds)

Could you elaborate on why this change is necessary?