feat: enable adding custom units of analysis (aois)

[theDebbister]

The MultiplEYE image generation generates aoi files automatically when generating the stimulus images which will be shown during the experiment. Per default, character-based aois are generated as well as white-space delimited word aois. For languages where white-space delimited words do not make sense (e.g., Chinese), there is the possibility to add manually defined aois. This PR integrates adding those custom aois. More specifically, the following features / changes are made:

• renaming of word column in the aoi files (and all word related columns, e.g., word_idx) --> into unit_of_analysis. This is to not restrict the primary aoi to be a "word" but it can also be a morpheme etc. This is implemented for all languages, no matter if they have custom aois
• refactor all methods using the former word column --> use units_of_analysis
• For languages adding custom aois: the unit_of_analysis (white-space delimited chars) is renamed into secondary_unit_of_analysis. The manually created custom aois are merged with the original aoi files in the column unit_of_analysis --> custom aois are by default used for scanpath mapping and reading measures if available!
• Users can specify the existence of the custom aois in the config file. If they are specified as existing they will be used as the default aoi.
• custom aois are expected to be found in a folder called custom_units_of_analysis in the data folder on SwitchDrive. The file format should follow the standard aoi file format.
• There are and will be language-specific scripts to add the custom aois