Feature Request: convert transistor files from different programs to another

[gituser789]

After importing scripts are working (e.g. see #16), there should be provided some functions to make life easy for the user to directly change from one file format to another.
Due to better overview and not overfilling the own database, a direct function might be useful.

tdb.plecs_to_geckocircuits(plecsfile.xml)
tdb.geckocircuits_to_plecs(geckofile.scl)
...
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