Further harmonize filename kwarg

[teutoburg]

Following AstarVienna/irdb#147, this further harmonizes file_name to filename and throw a warning if the old one is still used, as suggested by @hugobuddel in that same PR.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7cfe1f5) 75.14% compared to head (59d509e) 75.13%.

Files	Patch %	Lines
scopesim/effects/data_container.py	50.00%	1 Missing ⚠️
scopesim/effects/metis_lms_trace_list.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev_master     #361      +/-   ##
==============================================
- Coverage       75.14%   75.13%   -0.02%     
==============================================
  Files              58       58              
  Lines            8004     8006       +2     
==============================================
  Hits             6015     6015              
- Misses           1989     1991       +2     

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[hugobuddel]

Good! I I verified that we don't use file_name anywhere else for the file of an Effect.