Follow up to Issue103: clean up

[UWinch]

Is your feature request related to a problem? Please describe. Some issues from Issue 103 are still open:

• move list of supported UIMA file formats out of comments into rst and reference it in the method comments instead
• check if import_documentsuses a similar method as _create_text_analysis_request_parts, re-factor if true
• check for a better way to store process types than enum